The tunicates are the circulatory system. Subtype Shellers. The main features and structure of the ascidian. General characteristics of the chordate type
subject of zoology. Its position in the system of biological sciences.
Zoology (from other Greek ζῷον - animal and λόγος - teaching) is a biological science, the subject of which is representatives of the animal kingdom. Subject of zoology[edit]
Zoology studies the physiology, anatomy, embryology, ecology, and phylogeny of animals.
Animals traditionally included organisms with a certain set of characteristics:
1. Eukaryotic organisms.
2. The presence of an actin-myosin complex in cells (unlike plants and fungi).
3. Nutrition, as a rule, is associated with the absorption of food substrate particles by the body (unlike fungi).
4. There are no plastids (unlike plants).
5. Capable (as a rule) of active movement.
6. Store glycogen.
7. Chitin as the main component of the exoskeleton of many invertebrates (mainly arthropods; chitin is also formed in the organisms of many other animals - various worms, coelenterates, etc.).
Some simple organisms, according to the way of feeding and the set of subcellular structures, occupy an intermediate position between animals and plants and therefore can be considered both as objects of zoology and as objects of botany.
Zoology has a number of branches adjacent to other sciences.
Branches of zoology[edit]
According to the objectives of the study, zoology is divided into a number of basic disciplines, and according to the objects of study - into a number of auxiliary disciplines.
Core disciplines[edit]
The main disciplines of zoology, allocated according to the objectives of the study:
Systematics of animals.
Morphology of animals.
Embryology of animals.
Animal physiology.
Animal ethology.
Animal ecology.
Zoogeography.
Type chordates. General characteristics. Position in the system of the animal world and origin.
chordates
[edit]
From Wikipedia, the free encyclopedia
Chordates(lat. Chordata) - a type of deuterostomes, which are characterized by the presence of a mesodermal axial skeleton in the form of a chord, which in higher forms is replaced by a spine. According to the structure and function of the nervous system, the chordate type occupies the highest place among animals. More than 60,000 species of chordates are known in the world, in Russia - 4300 species.
concept chordates unites vertebrates and some invertebrates that have, at least during some period of their life cycle, a notochord, a hollow dorsal nerve cord, gill slits, an endostyle, and a tail located after the anus. The chordate type is divided into three subtypes: cephalochordates (lancelets), tunicates, vertebrates - the only subtype that has a skull. Previously, hemichordates were considered as the fourth subtype, which are now placed in a separate group.
Building features[edit]
Scheme of the structure of the cephalochords using the example of the Lancelet: 1 - thickening of the neural tube in front ("brain"); 2 - chord; 3 - dorsal nerve cord ("spinal cord"); 4 - tail fin; 5 - anus; 6 - digestive canal; 7 - circulatory system; 8 - outlet of the peribranchial cavity (atriopore); 9 - peribranchial cavity; 10 - pharyngeal (gill) slits; 11 - pharynx; 12 - oral cavity; 13 - near-mouth tentacles; 14 - mouth opening; 15 - sex glands (testes or ovaries); 16 - eyes of Hesse; 17 - nerves; 18 - metapleural fold; 19 - blind hepatic outgrowth. Respiration (gas exchange): the blue arrow indicates the inlet of water rich in oxygen, and the red arrow indicates the outlet enriched with carbon dioxide.
Chordates are a type of animals characterized by bilateral symmetry and the presence, at least at certain stages of development, of the following features:
Chord, which is an elastic rod of mesodermal origin. In vertebrates, the notochord during embryonic development is completely or partially replaced by cartilage and bone tissue that forms the spine.
The neural tube located dorsally. In vertebrates, the spinal cord develops into the brain.
Gill slits - paired openings in the pharynx. In lower chordates, they are involved in filtering water for food. In terrestrial vertebrates, gill slits are formed in early embryogenesis in the form of gill sacs.
Muscular tail - postanal part of the body, located caudal to the body of the anus, displaced to the ventral side (the chord and the neural tube enter it, but the intestine does not enter).
Endostyle - a groove on the ventral side of the pharynx. In the lower chordates, filter feeders, mucus is produced in it, which helps to collect food particles and deliver them to the esophagus. It also accumulates iodine and is possibly the precursor of the vertebrate thyroid gland. As such, only the gerbil has an endostyle in vertebrates.
Subtype Shellers. The main features and structure of the ascidian.
TYPE OF SHELL (TUNICATA)
Tunics, or tunicates, which include ascidians, pyrosomes, sebaceous and appendicularia, is one of the most amazing groups of marine animals. They got their name because their body is dressed on the outside with a special gelatinous shell, or tunic. The tunic is composed of a substance extremely similar in composition to cellulose, which is found only in the plant kingdom and is unknown to any other group of animals. Tunicates are exclusively marine animals, leading a partly attached, partly free-swimming pelagic lifestyle. They can be either solitary, or form amazing colonies that arise during the alternation of generations as a result of the budding of asexual single individuals. About the methods of reproduction of these animals - the most unusual among all living creatures on Earth - we will specifically discuss below.
Type Chordates
Inferior chordates. Subtype Cranial
TYPE CHORDS. LOWER CHORDS
General characteristics of the chordate type
Type Chordates combines animals diverse in appearance and lifestyle. Chordates are distributed throughout the world, have mastered a variety of habitats. However, all representatives of the type have the following common organizational features:
1. Bilaterally symmetrical chordates, deuterostomes, multicellular animals.
2. Chordates have a notochord throughout their life or at one of the phases of development. Chord- This is an elastic rod located on the dorsal side of the body and performing a supporting function.
3. Above the chord is located nervous system in the form of a hollow tube. In higher chordates, the neural tube is differentiated into the spinal cord and brain.
4. Under the chord is located digestive tube. The alimentary canal begins mouth and ends anus, or the digestive system opens into the cloaca. Throat pierced gill slits, which in primary aquatic organisms persist throughout their lives, while in terrestrial ones they are laid only at the early stages of embryonic development.
5. Beneath the digestive system lies a heart. The circulatory system in chordates closed.
6. Chordates have secondary body cavity.
7. Chordates are segmented animals. Location of organs metameric, i.e. the main organ systems are located in each segment. In higher chordates, metamerism is manifested in the structure of the spinal column, in the muscles of the abdominal wall of the body.
8. The organs of excretion in chordates are varied.
9. Chordates have separate sexes. Fertilization and development are varied.
10. Chordates descended through a series of intermediate forms unknown to biology from the very first coelomic animals.
The chordate type is divided into three subtypes:
1. Subtype Cranial. These are 30-35 species of small marine chordates, resembling fish in shape, but without limbs. The notochord in the Skullless persists throughout life. Nervous system in the form of a hollow tube. The pharynx has gill slits for breathing. Representatives - Lancelets.
2. Subtype Larval-chordaceae, or Shellers. These are 1500 species of marine sedentary sedentary animals living in tropical and subtropical regions. Their body is in the form of a bag (the body size of one individual in a colony is no more than 1 mm, and single ones can reach 60 cm), there are two siphons on the body - oral and cloacal. Larval chordates are water filterers. The body is covered with a thick shell - a tunic (hence the name of the subtype - Tunics). As adults, the tunicates lack the notochord and neural tube. However, the larva, which actively swims and serves for settling, has a structure typical of Chordates and is similar to the Lancelet (hence the second name - Larval Chordates). Representative - Ascidia.
3. Subtype Vertebrates, or cranial. These are the most highly organized chordates. Nutrition in vertebrates is active: food is searched for and pursued.
The notochord is replaced by the vertebral column. The neural tube is differentiated into the spinal cord and brain. The skull is developed, which protects the brain. The skull bears jaws with teeth for grasping and grinding food. Paired limbs and their belts appear. Cranials have a much higher level of metabolism, a complex population organization, diverse behavior, and a pronounced individuality of individuals.
The subtypes Cranial and Larval Chordates are called the lower Chordates, and the Vertebrate subtype is the higher Chordates.
Subtype Cranial - Acrania
Lancelet
The subtype Cranial includes the only class of the Head Chordidae, which has only about 30-35 species of marine animals that live in shallow water. A typical representative is Lancelet — Branchiostoma lanceolatum(Lancelet genus, class Headochord, subtype Cranial, type Chordata), the size of which reaches 8 cm. The body of the Lancelet is oval in shape, narrowed towards the tail, laterally compressed. Outwardly, the Lancelet resembles a small fish. Located on the back of the body tail fin in the form of a lancet - an ancient surgical instrument (hence the name Lancelet). Paired fins are absent. There is a small dorsal. On the sides of the body from the ventral side hang two metapleural folds, which fuse on the ventral side and form peribranchial, or the atrial cavity, which communicates with the pharyngeal fissures and opens at the posterior end of the body with a hole - atriopore- outside. At the anterior end of the body near the mouth are the perioral tentacles, with which the Lancelet captures food. Lancelets live on sandy soils in the sea at a depth of 50-100 cm in temperate and warm waters. They feed on bottom sediments, marine ciliates and rhizopods, eggs and larvae of small marine crustaceans, diatoms, burrowing into the sand and exposing the front end of the body. More active at dusk, avoid bright lighting. Disturbed Lancelets swim quite quickly from place to place.
Covers. The body of the lancelet is covered skin, consisting of a single layer epidermis and thin layer dermis.
Musculoskeletal system. A chord stretches along the entire body. Chord- this is an elastic rod located on the dorsal side of the body and performing a supporting function. To the anterior and posterior ends of the body, the chord becomes thinner. The notochord protrudes into the anterior part of the body a little further than the neural tube, hence the name of the class - Cephalic. The notochord is surrounded by connective tissue, which simultaneously forms supporting elements for the dorsal fin and divides the muscle layers into segments using connective tissue
Type Chordates subtype Cranial Lancelet
layers. Individual muscle segments are called myomers, and the partitions between them myoseptami. Muscles are formed by striated muscles.
body cavity at the lancelet secondary in other words, they are coelomic animals.
Digestive system. On the front of the body is mouth opening, surrounded by tentacles(up to 20 pairs). The mouth opening leads to a large throat, which functions as a filtering apparatus. Through the cracks in the pharynx, water enters the atrial cavity, and food particles are directed to the bottom of the pharynx, where endostyle- a groove with a ciliated epithelium that drives food particles into the intestine. no stomach, but hepatic outgrowth, homologous to the liver of vertebrates. midgut, without making loops, opens anus at the base of the tail fin. Digestion of food occurs in the intestines and in the hollow hepatic outgrowth, which is directed towards the head end of the body. Interestingly, the Lancelet retained intracellular digestion, intestinal cells capture food particles and digest them in their digestive vacuoles. This mode of digestion is not found in vertebrates.
Respiratory system. There are more than 100 pairs in the throat of the Lancelet gill slits leading to peribranchial cavity. The walls of the gill slits are penetrated by a dense network of blood vessels in which gas exchange occurs. With the help of the ciliary epithelium of the pharynx, water is pumped through the gill slits into the peribranchial cavity and through the opening (atriopore) is brought out. In addition, gas-permeable skin also takes part in gas exchange.
Circulatory system. The circulatory system of the Lancelet closed. The blood is colorless and contains no respiratory pigments. The transport of gases is carried out as a result of their dissolution in the blood plasma. In the circulatory system one circle circulation. The heart is absent, and blood is moved by the pulsation of the gill arteries, which pump blood through the vessels in the gill slits. Arterial blood enters dorsal aorta, from which carotid arteries blood flows to the front, and through the unpaired dorsal aorta to the back of the body. Then by veins blood returns to venous sinus and by abdominal aorta heading for the gills. All blood from the digestive system enters the hepatic outgrowth, then into the venous sinus. The liver outgrowth, like the liver, neutralizes toxic substances that have entered the bloodstream from the intestines, and, in addition, performs other functions of the liver.
Such a structure of the circulatory system does not fundamentally differ from the circulatory system of vertebrates and can be considered as its prototype.
excretory system. The excretory organs of the lancelet are called nephridia and resemble the excretory organs of flatworms - protonephridia. Numerous nephridia (about a hundred pairs, one for two gill slits), located in the pharynx, are tubules that open with one hole into the coelom cavity, with the other into the paragillary cavity. On the walls of the nephridium are club-shaped cells - solenocytes, each of which has a narrow channel with a ciliated hair. Due to the beating of these
Type Chordates subtype Cranial Lancelet
hairs, the liquid with metabolic products is removed from the cavity of the nephridium into the peribranchial cavity, and from there it is already out.
central nervous system formed neural tube with a cavity inside. The lancelet does not have a pronounced brain. In the walls of the neural tube, along its axis, there are light-sensitive organs - eyes Hesse. Each of them consists of two cells - photosensitive and pigmented, they are able to perceive the intensity of light. An organ adjacent to the expanded anterior part of the neural tube smell.
Reproduction and development. The lancelets that live in our Black Sea and the lancelets that live in the waters of the Atlantic off the coast of Europe break into breeding in the spring and spawn eggs until August. Warm water lancelets breed all year round. lancelets separate sexes, sex glands (gonads, up to 26 pairs) are located in the body cavity in the pharynx. Sexual products are excreted into the peribranchial cavity through the temporarily formed genital ducts. Fertilization external in water. emerges from the zygote larva. The larva is small: 3-5 mm. The larva actively moves with the help of cilia that cover the entire body, and due to the lateral bends of the body. The larva swims in the water column for about three months, then passes to life at the bottom. Lancelets live up to 4 years. Sexual maturity is reached by two years.
Significance in nature and for man. The non-cranial are an element of biological diversity on Earth. They feed on fish and crustaceans. The Skullless themselves process dead organic matter, being decomposers in the structure of marine ecosystems. The non-cranial are essentially a living blueprint for the structure of chordate animals. However, they are not direct ancestors of vertebrates. In the countries of Southeast Asia, local residents collect Lancelets by sifting sand through a special sieve and eat them.
Non-cranial animals have retained a number of features characteristic of their invertebrate ancestors:
§ excretory system of nephridial type;
§ the absence of differentiated sections in the digestive system and the preservation of intracellular digestion;
§ filtering method of nutrition with the formation of a near-gill cavity to protect the gill slits from clogging;
§ metamerism (repetitive arrangement) of the genital organs and nephridia;
§ absence of a heart in the circulatory system;
§ weak development of the epidermis, it is single-layer, like in invertebrates.
Type Chordates subtype Cranial Lancelet
Rice. The structure of the lancelet.
A - neural tube, chord and digestive system; B - circulatory system.
1 - chord; 2. - neural tube; 3 - oral cavity; 4 - gill slits in the pharynx; 5 - peribranchial cavity (atrial cavity); 6 - atriopore; 7 - hepatic outgrowth; 8 - gut; 9 - anus; 10 - subintestinal vein; 11 - capillaries of the portal system of the hepatic outgrowth; 12 - abdominal aorta; 13 - pulsating bulbs of the arteries pumping blood through the gill slits; 14 - dorsal aorta.
Rice. Nephridium Lancelet.
1 - hole as a whole (into the secondary cavity of the body); 2 - solenocytes; 3 - opening into the circumbranchial cavity.
Type Chordates subtype Cranial Lancelet
Rice. Cross section of the Lancelet:
A - in the region of the pharynx, B - in the region of the midgut.
1 - neural tube; 2 - muscles; 3 - roots of the dorsal aorta; 4 - ovary; 5 - endostyle; 6 - abdominal aorta; 7 - metapleural folds; 8 - peribranchial (atrial) cavity; 9 - gill slits (due to the oblique position, more than one pair is visible on one transverse section); 10 - nephridia; 11 - whole; 12 - ventral (motor) spinal nerve; 13 - dorsal (mixed) nerve; 14 - chord; 15 - subintestinal vein; 16 - dorsal aorta; 17 - dorsal fin.
Questions for self-control.
Name the characteristic features of animals of the Chordata type.
Name the type classification into three subtypes.
Name the systematic position of the Lancelet.
Where does the lancelet live?
What is the body structure of the Lancelet?
How does the Lancelet eat and what is the structure of the digestive system of the Lancelet?
How is the excretion of waste products from the Lancelet?
What is the structure of the nervous system of the Lancelet?
What is the structure of the circulatory system of the Lancelet?
How does the lancelet reproduce?
What is the significance of the Lancelet in nature?
PICTURES TO BE COMPLETED IN THE ALBUM
(total 3 drawings)
Lesson topic:
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chordates
Chordates are the highest phylum of deuterostomes. All species of this type are characterized at least at the stage of embryonic development by the presence of an unsegmented dorsal skeletal axis (chord), dorsal neural tube, and gill slits.
Type Chordata. General characteristics. Structural features
The type is divided into three subtypes: tunicates, non-cranial and vertebrates.
Tunicates (Tunicata) or larval-chords (Urochordata) have a bag-shaped or barrel-shaped body from 0.3 to 50 cm long; the size of a colony of pyrosomes can exceed 30 m. The body of the tunicates is enclosed in a gelatinous tunic secreted by the outer epithelium.
The pharynx is pierced by gill slits. The hindgut and gonadal ducts open into the atrial cavity, which is connected to the external environment. The nervous system consists of a ganglion located between the mouth and the atriopore, with a nerve trunk extending from it; sense organs are poorly developed.
Tunicates reproduce sexually; asexual reproduction also occurs. All larvae are marine animals that feed on algae, small animals and detritus.
In contrast to the simplified structure of adult forms leading a sedentary lifestyle, the larvae are active, have developed sensory organs and a nervous system, muscles and a notochord (in adult forms, it remains only in appendicularia). Vertebrates are believed to have descended from neotenic (starting to breed) tunicate larvae. Three classes: tiny primitive appendiculars (Appendicularia), sea squirts (Ascidiacea) and pelagic tunicates (Thaliacea), including three subclasses: pyrosomes, salps and casks.
About 3000 species, mainly in the upper layers of the seas and oceans.
Cranial (Acrania) or cephalochord (Cephalochordata) - a subtype of lower chordates.
The head is not isolated, the skull is absent (hence the name). The entire body, including some internal organs, is segmented. Respiratory organs - gills. Blood moves due to the pulsating abdominal vessel. The sense organs are represented only by feeling cells.
The subtype includes two families (about 20 species), whose representatives live in temperate and warm seas; the most famous is the lancelet.
Vertebrates (Vertebrata) or cranial (Craniota) are the most highly organized group of animals.
Vertebrates, for example, lose to insects in terms of the number of species, but they are very important for the modern biosphere, since they usually complete all food chains.
Due to the presence of a complex nervous system and the ability to live in a wide variety of conditions, vertebrates were divided into sharply different systematic groups and managed to achieve not only high perfection in morphology, physiology and biochemistry, but also the ability to higher forms of behavior and mental activity.
The main features of vertebrates: the presence of a notochord in an embryo, which in an adult animal transforms into a spine, an internal skeleton, a separate head with a developed brain, a protected skull, perfect sensory organs, developed circulatory, digestive, respiratory, excretory and reproductive systems.
Vertebrates reproduce exclusively sexually; most of them are dioecious, but some fish are hermaphrodites.
The first vertebrates appeared in the Cambrian. 8 classes combined into 2 superclasses: jawless (Agnatha) - scutellous and cyclostomes and jawed (Gnathostomata) - armored, cartilaginous and bone fish, amphibians, reptiles, birds, mammals. Shield fish, as well as armored fish, became extinct in the Paleozoic. About 50,000 species of vertebrates are currently known.
General characteristics of the chordate type
The main terms and concepts tested in the examination paper: non-cranial, gill slits, internal skeleton, amphibians, skin, limbs and limb girdle, circulation, lancelet, mammals, neural tube, vertebrates, reptiles, birds, reflexes, adaptations to lifestyle, fish, bone skeleton, cartilaginous skeleton, notochord .
To type Chordates include animals that have an internal axial skeleton - a chord or a vertebral column.
Chordate animals have reached in the process of evolution the highest, in comparison with other types, the level of organization and flourishing. They live in all areas of the globe and occupy all habitats.
chordates are bilaterally symmetrical animals with a secondary body cavity and a secondary mouth.
In chordates, there is a general plan for the structure and location of internal organs:
- the neural tube is located above the axial skeleton;
- under it is a chord;
- under the chord is the digestive tract;
- under the digestive tract - the heart.
In the phylum Chordates, two subtypes are distinguished - Cranial and Vertebrate.
Refers to the non-skull lancelet. All other chordates known today, considered in the school biology course, belong to the subtype Vertebrates.
The subtype Vertebrates includes the following classes of animals: Fish, Amphibians, Reptiles, Birds, Mammals.
General characteristics of chordates.Skin vertebrates protect the body from mechanical damage and other environmental influences.
The skin is involved in gas exchange and excretion of decay products.
Derivatives of the skin are hair, claws, nails, feathers, hooves, scales, horns, needles, etc. Sebaceous and sweat glands develop in the epidermis.
Skeleton, representatives of the chordate type can be connective tissue, cartilaginous and bone. The non-cranial have a connective tissue skeleton. In vertebrates - cartilaginous, bone-cartilaginous and bone.
musculature- divided into striated and smooth.
The striated muscles are called skeletal. Smooth muscles form the muscular system of the jaw apparatus, intestines, stomach and other internal organs. The skeletal muscles are segmented, although less than in lower vertebrates. Smooth muscle has no segmentation.
Digestive system It is represented by the oral cavity, pharynx, always associated with the respiratory organs, esophagus, stomach, small and large intestines, digestive glands - the liver and pancreas, which develop from the wall of the anterior intestine.
In the process of evolution of chordates, the length of the digestive tract increases, it becomes more differentiated into sections.
Respiratory system formed by gills (in fish, amphibian larvae) or lungs (in terrestrial vertebrates).
The skin serves as an additional respiratory organ for many. The gill apparatus communicates with the pharynx. In fish and some other animals, it is formed by the gill arches, on which the gill filaments are located.
The lungs during embryonic development are formed from outgrowths of the intestine and are of endodermal origin.
The circulatory system is closed. The heart consists of two, three or four chambers. Blood enters the atria, and is sent to the bloodstream by the ventricles.
There is one circulation circle (in fish and amphibian larvae) or two (in all other classes). The heart of fish, amphibian larvae is two-chambered. Adult amphibians and reptiles have a three-chambered heart. However, reptiles develop an incomplete interventricular septum. Fish, amphibians and reptiles are cold-blooded animals.
Birds and mammals have a four-chambered heart. These are warm-blooded animals.
Blood vessels are divided into arteries, veins and capillaries.
Nervous system ectodermal origin. It is laid in the form of a hollow tube on the dorsal side of the embryo. The central nervous system is made up of the brain and spinal cord. The peripheral nervous system is made up of cranial and spinal nerves and interconnected ganglia along the spinal column.
Spinal cord is a long cord lying in the spinal canal. Spinal nerves branch off from the spinal cord.
sense organs well developed. Primitive aquatic animals have organs sideline, perceiving pressure, direction of movement, speed of water flow.
excretory organs all vertebrates are represented by kidneys. The structure and mechanism of functioning of the kidneys changes in the process of evolution.
Reproductive organs. Vertebrates are dioecious.
The sex glands are paired and develop from the mesoderm. The genital ducts are connected with the excretory organs.
Superclass Pisces
Fish appeared in the Silurian - Devonian from jawless ancestors.
There are about 20,000 species. Modern fish are divided into two classes - cartilaginous and Bone. Cartilaginous fish include sharks and rays, characterized by a cartilaginous skeleton, the presence of gill slits, and the absence of a swim bladder.
Characteristics of the chordate type (Chordata)
Bony fish include animals that have bony scales, a bone skeleton, gill slits covered with a gill cover. The appearance of fish is due to the following aromorphoses :
- the appearance of a cartilaginous or bone spine and a skull that covers the spinal cord and brain from all sides;
- the appearance of the jaws;
- the appearance of paired limbs - ventral and pectoral fins.
All fish live in water, have a streamlined body, divided into a head, body and tail.
The sense organs are well developed - sight, smell, hearing, taste, organs of the lateral line, balance. The skin is two-layered, thin, mucous, covered with scales. The muscles are almost undifferentiated, with the exception of the muscles of the jaws and the muscles attached to the gill covers of bony fish.
Digestive system well differentiated into departments.
There is a liver with a gallbladder and a pancreas. Many have developed teeth.
Respiratory organs fish have gills, and lungfish have gills and lungs. An additional function of breathing is performed by the swim bladder in bony fish. It also performs a hydrostatic function.
Circulatory system closed. One circle of blood circulation. The heart consists of an atrium and a ventricle.
Venous blood from the heart through the afferent branchial arteries enters the gills, where the blood is saturated with oxygen. Arterial blood flows through the efferent branchial arteries into the dorsal aorta, which supplies blood to the internal organs.
Fish have a portal system of the liver and kidneys, which cleans the blood of harmful substances. Fish are cold-blooded animals.
excretory system represented by ribbon-like primary kidneys. Urine flows through the ureters to the bladder. In males, the ureter is also the vas deferens.
Females have an independent excretory opening.
gonads represented by paired testes in males and ovaries in females. Many fish show sexual dimorphism. Males brighter than females attract them with their appearance, mating dances.
In the nervous system the development of the diencephalon and midbrain should be noted.
Most fish have a well-developed cerebellum, which is responsible for coordinating movements and maintaining balance. The forebrain is less developed than in the higher classes of animals.
Eyes have a flat cornea, a spherical lens.
hearing organs represented by the inner ear - the membranous labyrinth. There are three semicircular canals.
They contain lime stones. Fish make and pick up sounds.
sense organs represented by sensitive cells scattered throughout the body.
Lateral line perceives the direction of flow and water pressure, the presence of obstacles, sound vibrations.
taste cells are in the oral cavity.
The value of fish in nature and human life. Consumers of plant biomass, consumers of the second and third orders; food sources, fats, vitamins.
EXAMPLES OF TASKS
Part A
The non-skull animals are
3) lancelet
4) octopus
A2. The main feature of chordates is
1) closed circulatory system
2) internal axial skeleton
3) gill breathing
4) striated muscles
A3. The bone skeleton is
1) white shark 3) stingray
2) katrana 4) piranhas
A4. Warm-blooded animals include
1) whale 2) sturgeon 3) crocodile 4) toad
There are bony gill covers
1) dolphin 3) tuna
2) sperm whale 4) electric stingray
Have a four-chambered heart
1) turtles 2) pigeons 3) perches 4) toads
1) single chamber heart and two circles of blood circulation
2) two-chambered heart and one circle of blood circulation
3) three-chambered heart and one circle of blood circulation
4) two-chambered heart and two circles of blood circulation
A8. Cold-blooded animals are
1) beaver 3) squid
2) sperm whale 4) otter
The coordination of fish movements is regulated
1) forebrain 3) spinal cord
2) midbrain 4) cerebellum
A10. No swim bladder
1) katrans 2) pike 3) perch 4) sturgeon
Part B
IN 1. Choose the right statements
1) fish have a three-chambered heart
2) the transition of the head to the trunk in fish is clearly visible
3) there are nerve endings in the organs of the lateral line of fish
4) the chord in some fish lasts a lifetime
5) fish are not capable of forming conditioned reflexes
6) the nervous system of fish consists of the brain, spinal cord and peripheral nerves
Select the features related to non-cranial animals
1) the brain is not differentiated into sections
2) the internal skeleton is represented by a chord
3) excretory organs - kidneys
4) the circulatory system is not closed
5) the organs of vision and hearing are well developed
6) the pharynx is pierced by gill slits
VZ. Establish a correspondence between the signs of animals and the type to which these animals belong.
Part C
Where can deep sea fish store oxygen? Why do they need to do this?
C2. Read the text carefully. Indicate the numbers of sentences in which errors were made. Explain and correct them.
1. Type of chordates - one of the largest in terms of the number of species in the animal kingdom. 2. The internal axial skeleton in all representatives of this type is the chord - a bone, dense, elastic strand 3. The Chordata type is divided into two subtypes - Vertebrates and Invertebrates.
4. In the nervous system, the anterior part of the brain is most developed. 5. All chordates have radial symmetry, a secondary body cavity, and a closed circulatory system. 6. An example of primitive chordates is the lancelet.
hullers They represent a sharply isolated group, differing in their organization and way of life. These are marine solitary or colonial animals leading a sedentary (attached to the substrate) or sedentary lifestyle.
Typical signs of chordates are clearly expressed only on larval stage. The body shape is sac-shaped or barrel-shaped. Outside, the body is covered with a special shell - tunic containing fiber-like substance - tunicin(this is the only case in the animal world of the formation of a substance close to plant fiber).
The notochord is present only in the larval state, with the exception of appendicularia, in which the remnants of the notochord persist for life. There is no tubular nervous system in adult tunicates. They feed passively, filtering large masses of water. The circulatory system is open, lacunar type.
They reproduce both sexually (tunicates are hermaphrodites) and asexually (by budding).
The subtype includes three classes: Ascidia (Ascidiae), Salp (Salpae) and Appendicular (Appendiculariae).
Ascidian class includes about 1 thousand species of single or colonial marine animals. Most adults lead a sedentary lifestyle; larvae are free-living. Outwardly resemble double jar, attached by the base to the substrate and having two holes in the upper part of the body - oral and cloacal siphons.
Outside, the body is covered with a tunic secreted by the epithelium, which is saturated with inorganic salts, which turns it into a dense protective shell.
Under the tunic lies skin-muscular sac or mantle. Water is pumped into the pharynx by contraction and relaxation of the mantle muscles, as well as by the flickering of the cilia of the epithelium of the inner walls of the oral siphon.
Siphons have special annular bundles of muscles that close and open these openings.
The pharynx of ascidians occupies a large part of the body, its walls are pierced by many holes-stigmas that open into a special circumbranchial cavity that encloses the pharynx. Ascidians, like lancelets, have an endostyle in the pharynx, the mucus of which traps food particles from water entering through the oral siphon. Power is passive (filtered). Food particles enter the esophagus, then into the stomach, where digestion and absorption take place, undigested residues through the anus with a stream of water are removed from the body through the cloacal siphon. The pharynx also serves as a respiratory organ, gas exchange occurs in the vessels braiding the pharynx.
The heart looks like a short tube and is located on the ventral side of the body near the stomach. From the anterior end of the heart, a vessel departs, carrying blood to the walls of the pharynx. The vessel extending from the posterior end branches and approaches the internal organs (stomach, intestines, genital organs) and the mantle, where it flows into small gaps located between the organs. Circulatory system open. The heart pulsates so that the blood rushes out of it alternately either towards the pharynx, where it is saturated with oxygen, or in the opposite direction. Thus, in ascidians, the same vessels are either arteries or veins.
The nervous system of an adult ascidian is represented by nerve ganglion(devoid of an internal cavity), located near the oral siphon.
All sea squirts - hermaphrodites. The sex glands are located near the stomach. The ducts of the glands flow into the peribranchial cavity. Sexual products through the cloacal siphon are excreted into the environment. Fertilization occurs either in the peribranchial cavity, where the reproductive products of another individual enter with a current of water, or in the external environment. Self-fertilization does not occur, since eggs and sperm mature in one individual at different times. Ascidians reproduce both sexually and asexually. The embryonic development of ascidians is of great general biological interest, since its knowledge made it possible to establish the true position of the tunicates in the animal system and the undoubted belonging to chordates, because it is the larva that has all the typical signs of chordates.
In the process of development of a fertilized egg, a tailed larva is formed, outwardly similar to a tadpole, it leads a free-swimming lifestyle and sharply differs in structure from adult ascidians. It has an oval body and a long tail. The nervous system is represented by the neural tube, which has an extension of the neurocoel in the head section - the cerebral vesicle,
Where are the pigmented eye and statocyst located? The larva has a notochord - an elastic cord of highly vacuolated cells, located, like in all typical chordates, under the neural tube. Muscle cells lie on the sides of the notochord.
Asexual reproduction is carried out by budding.
Salpa class - free-swimming marine animals, in the world fauna there are about 25 species. The sizes of single individuals are from a few millimeters to 5-15 cm. The length of polymorphic colonies of barrel bugs can reach 30-40 cm. They have structural features in common with ascidians, but differ in their ability to jet propulsion. The body resembles a barrel, oral and cloacal siphons are located at opposite ends of the body. The tunic is thin, transparent, so that the internal organs are clearly visible through it.
The mantle is formed by a single-layer epithelium: the musculature is arranged in the form tapes which, like hoops, encircle the body salps. With the successive contraction of the muscle bands, water is pushed out of the cloacal siphon and imparts a forward movement to the animal. In the adult state, there is no notochord. Salps are characterized by alternation of sexual and asexual generations (metagenesis). Fertilized eggs produce asexual salps that reproduce by budding. Budding individuals form gonads and reproduce sexually. There are no free-swimming larvae characteristic of ascidians.
Appendicular class unites about 60 species of small tunicates with a body length of several millimeters; only some species reach a length of 1-2 cm. Appendicularia lead a free-floating lifestyle. Representatives of this class are the least in comparison with other Shellers evade typical chordates.
The appearance and internal structure resemble ascidian larvae, differing only in details. Appendicularia have an oval body with a long, compressed tail. Throughout their lives they have chord, covered with connective tissue. The chord runs from the base to the end of the tail. Above the chord lies the nerve trunk, and on the sides - two muscle cords
The nervous system consists of the nerve ganglion, from which the nerve trunk departs, passing along the dorsal side of the tail.
There are statocysts. Gill openings two. There is no peritoneal cavity.
On the ventral side of the body lies a small heart, making up to 250 contractions per minute.
There is no real tunic in appendiculars. The animal is surrounded by a gelatinous "house", from which the appendicularia emerges several times a day, destroying its walls with its tail. The front of the house has a hole covered with a grate of thickened filaments of slime. Inside the house there is a "trapping net" of thin elongated formations, the mouth of the animal is turned to its top. The “house” of the appendicularium is formed by products of the secretion of the skin epithelium containing chitin-like substances.
They reproduce only sexually, without a distinct larval stage.
(UROCHORDATA, or TUNICATA)
Tunicates are widely distributed in the oceans and seas. There are about 1100 species of them, of which about 1000 belong to the class of ascidians, leading an attached lifestyle. Most ascidians are solitary animals, the rest form colonies.
The body is covered with a thick shell - a tunic (which explains one of the names of the subtype), which forms a bag that communicates with the external environment with two wide tubes (siphons). Water enters the body through one of them, and it exits through the other (Fig. 68). The usual body size is a few centimeters.
The nervous system is poorly developed. It is represented by a small ganglion lying above the pharynx, and nerves extending from it to various organs. There is a thin skin-muscular sac.
The digestive system begins with the mouth, which communicates with the external environment through the inlet siphon, and consists of the pharynx (there is an endostyle on its dorsal side), the stomach, and the horseshoe-shaped intestine, which opens with an anus into the outlet siphon. The pharynx is pierced by small gill openings that open into the peribranchial cavity. Reception of food (small organisms and organic pieces) and its digestion occurs, as in lancelets.
Rice. 68. Ascidia:
//- appearance, //- internal structure; 7 - oral siphon; 2- cloacal siphon; 3 - tunic (sheath); 4, 5 - mantle; 6 - pharynx; 7 - pharyngeal cavity; 8 - gill openings; 9 - endostyle; 10, 11 - peribranchial cavity; 12 - its wall; 13 - stomach; 14 - hepatic outgrowth; 15 - anus; 16 - testis; 17 - ovary; 18 - ducts of the sex glands; 19 - pericardium; 20 - a heart; 21 - ganglion
The circulatory system is not closed. Blood is set in motion by the heart, from which vessels depart to various organs, especially strongly branched in the walls of the gill slits of the pharynx. The latter is very large and, like in lancelets, plays the role of a respiratory organ through which water passes, which is removed after gas exchange through the outlet siphon.
Dissimilation products are accumulated by some cells and remain in the body.
All tunicates are hermaphrodites; fertilization external and internal. Many species also reproduce asexually (by budding).
The position of tunicates in the animal system remained unclear for a long time, until A. O. Kovalevsky studied in detail the development of ascidians, showing that it is very similar to the development of lancelets and ends with the formation of a planktonic larva, similar in body shape to tadpoles and moving with the help of a tail. The larvae have a well-developed neural tube and notochord. After a short period of planktonic life, the larvae attach themselves to a solid substrate and their organization undergoes a radical restructuring, mostly regressive: the tail together with the neural tube (except for its anterior end, which turns into a ganglion) and the chord are reduced (as superfluous during a sedentary lifestyle), others the organs necessary for adult animals develop. The tunicates, thanks to their well-developed filtration apparatus, have become a large group that obtains their food in any place of the oceans and seas. The subtype is divided into 3 classes: ascidians, salps and appendiculars.
Ascidian class (ASCIDIAE)
The class includes about 1 thousand species. They are solitary or colonial sessile animals.
The body resembles a two-necked jar, attached by the base to the substrate and having two openings - oral and cloacal siphons. Outside, the body is covered with a tunic containing a fiber-like substance (the only case in the animal world of the formation of a substance close to cellulose). The tunic is secreted by the epithelium. Beneath it lies a skin-muscular sac, or mantle. The contraction or relaxation of the mantle muscles, together with the beating of the cilia of the epithelium of the inner walls of the oral siphon, contributes to the injection of water into the pharynx.
From the oral siphon, water enters the pharynx, which occupies most of the body. The walls of the pharynx are permeated with many holes - stigmas that open into the atrial cavity. The pharynx is followed by the esophagus, stomach, and intestine, which opens through the anus into the atrial cavity near the cloacal siphon. Passive power supply (filtering).
The pharynx also serves as a respiratory organ. The heart contracts now in one direction, then in the other direction with the same vessel. Like all tunicates, ascidians are hermaphrodites, but the sex glands do not develop simultaneously, and the same organism functions either as a male or as a female. Fertilization is external, rarely in the cloacal cavity.
The development of a fertilized egg leads to the formation of a tailed larva.
Salpa class (SALPAE, or THALIACEA)
The class includes 25 species that live in warm seas. Representatives outwardly resemble a barrel, in which the oral and cloacal siphons are located at opposite ends of the body (Fig. 69). They swim due to jet propulsion: water is pushed out of the cloacal opening with force, due to which the animal moves in jerks. There is an alternation of asexual and sexual generations (metagenesis). Fertilized eggs produce asexual salps that reproduce by budding. Budding individuals form gonads and reproduce sexually. There is no dispersal larva characteristic of ascidians.
Tunics, or tunicates, which include ascidians, pyrosomes, sebaceous and appendicularia, is one of the most amazing groups of marine animals. They got their name because their body is dressed on the outside with a special gelatinous shell, or tunic. The tunic is composed of a substance extremely similar in composition to cellulose, which is found only in the plant kingdom and is unknown to any other group of animals. Tunicates are exclusively marine animals, leading a partly attached, partly free-swimming pelagic lifestyle. They can be either solitary, or form amazing colonies that arise during the alternation of generations as a result of the budding of asexual single individuals. About the methods of reproduction of these animals - the most unusual among all living creatures on Earth - we will specifically discuss below.
The position of the tunicates in the system of the animal kingdom is very interesting. The nature of these animals remained mysterious and incomprehensible for a long time, although they were known to Aristotle more than two and a half thousand years ago under the name Tethya. Only at the beginning of the 19th century it was established that the solitary and colonial forms of some tunicates - salps - represent only different generations of the same species. Until then, they were classified as different types of animals. These forms differ from each other not only in appearance. It turned out that only the colonial forms have sexual organs, and the solitary forms are asexual. The phenomenon of alternation of generations in salps was discovered by the poet and naturalist Albert Chamisso during his voyage in 1819 on the Russian warship Rurik under the command of Kotzebue. Old authors, including Carl Linnaeus, attributed single tunics to the type of molluscs. Colonial forms were attributed by him to a completely different group - zoophytes, and some considered them a special class of worms. But in fact, these superficially very simple animals are not as primitive as they seem. Thanks to the work of the remarkable Russian embryologist A. O. Kovalevsky in the middle of the last century, it was established that tunicates are close to chordates. A. O. Kovalevsky established that the development of ascidia follows the same type as the development of the lancelet, which, according to the apt expression of Academician I. I. Shmalhausen, “is like a living simplified scheme of a typical chordate animal.” The group of chordates is characterized by a number of certain important structural features. First of all, this is the presence of a dorsal string, or chord, which is the internal axial skeleton of the animal. The larvae of the tunicate, floating freely in the water, also have a dorsal string, or chord, which completely disappears when they turn into an adult. The larvae are also much higher than the parental forms in terms of other important features of the structure. For phylogenetic reasons, i.e., for reasons connected with the origin of the group, greater importance is attached to the organization of their larvae in tunicates than to the organization of adult forms. Such an anomaly is unknown for any other type of animal. In addition to the presence of a notochord, at least in the larval stage, a number of other features bring together tunicates with true chordates. It is very important that the nervous system of the tunicates is located on the dorsal side of the body and is a tube with a channel inside. The neural tube of the tunicates is formed as a groove-like longitudinal protrusion of the surface integuments of the body of the embryo, the ectoderm, as is the case in all other vertebrates and in humans. In invertebrates, the nervous system always lies on the ventral side of the body and is formed in a different way. The main vessels of the circulatory system of tunicates, on the contrary, are located on the ventral side, in contrast to what is characteristic of invertebrates. And finally, the anterior intestine, or pharynx, is pierced by numerous holes in the tunicates and has become a respiratory organ. As we have seen in other chapters, the respiratory organs of invertebrates are very diverse, but the intestines never form gill slits. This is a sign of chordates. The embryonic development of the tunic also shares many similarities with the development of the Chordata.
At present, it is believed that tunicates, through secondary simplification, or degradation, originated from some forms very close to vertebrates.
Together with other chordates and echinoderms, they form the trunk of deuterostomes - one of the two main trunks of the evolutionary tree.
Shellers are considered either as a separate subphylum of chordate phylum- Chordata, which together with them include three more subtypes of animals, including vertebrates (Vertebrata), or as an independent type - Tunicata, or Urochordata. This type includes three classes: Appendicularia(Appendiculariae, or Copelata), sea squirts(Ascidiae) and salupy(Salpae).
Before ascidian divided into three groups: simple or solitary, ascidian (Monascidiae); complex or colonial, sea squirts (Synascidiae) and pyrosomes, or fireballs(Ascidiae Salpaeformes, or Pyrosomata). However, at present, the division into simple and complex ascidians has lost its systematic significance. Ascidians are divided into subclasses according to other characteristics.
Salps are divided into two groups - barrel makers(Cyclomyaria) and salp proper(Desmomyaria). Sometimes these units are given the meaning of subclasses. The salps apparently also include a very peculiar family of deep-sea bottom tunicates - Octacnemidae, although until now most authors considered it a strongly deviated subclass of ascidians.
Very often free-swimming salps and pyrosomes are united in the group of pelagic tunicates Thaliacea, which is given the significance of a class. The class Thaliacea is then divided into three subclasses: Pyrosomida or Luciae, Desmomyaria or Salpae, and Cyclomyaria or Doliolida. As can be seen, the views on the taxonomy of the higher groups of Tunicata are very different.
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Currently, more than a thousand species of tunicates are known. The vast majority of them fall to the share of ascidians, there are about 60 species of appendicularia, about 25 species of salps and approximately 10 species of pyrosomes (Tables 28-29).
As already mentioned, tunicates live only in the sea. Appendicularium, salps and pyrosomes swim in the ocean waters, while ascidians lead an attached lifestyle at the bottom. Appendicularia never form colonies, while salps and ascidians can occur both in the form of single organisms and in the form of colonies. Pyrosomes are always colonial. All tunicates are active filter feeders, feeding on either microscopic pelagic algae and animals, or particles of organic matter suspended in water - detritus. Driving water through the pharynx and out through the gills, they filter out the smallest plankton, sometimes using very complex devices.
Pelagic tunicates live mainly in the upper 200 m of water, but sometimes they can go deeper. Pyrosomes and salps are rarely found deeper than 1000 m, appendicularians are known up to 3000 m. At the same time, special deep-sea species are apparently absent among them. Ascidians in their bulk are also distributed in the tidal littoral and sublittoral zones of the oceans and seas - up to 200-500 m, however, a significant number of their species are also found deeper. The maximum depth of their location is 7230 m.
Tunicates are found in the ocean sometimes in single specimens, sometimes in the form of colossal clusters. The latter is especially characteristic of pelagic forms. In general, tunicates are quite common in the marine fauna and, as a rule, are caught in plankton nets and bottom trawls of zoologists everywhere. Appendicularia and sea squirts are common in the oceans at all latitudes. They are just as characteristic of the seas of the Arctic Ocean and Antarctica as they are of the tropics. Salps and pyrosomes, on the contrary, are mainly confined in their distribution to warm waters and are only occasionally found in waters of high latitudes, mainly being brought there by warm currents.
The structure of the body of almost all tunicates is unrecognizably very different from the general plan of the body structure in the type of chordates. Closest to the original forms are the appendiculars, and they occupy the first place in the tunic system. However, despite this, the structure of their body is the least characteristic of tunicates. Acquaintance with tunicates, apparently, is best to start with ascidia.
The structure of the ascidian.
Ascidians are benthic animals leading an attached lifestyle. Many of them are single forms. The size of their body averages a few centimeters in diameter and the same in height. However, some species are known among them, reaching 40-50 cm, for example, the widespread Cione intestinalis or the deep-sea Ascopera gigantea. On the other hand, there are very small ascidians, less than 1 mm in size. In addition to solitary ascidians, there are a large number of colonial forms in which individual small individuals, a few millimeters in size, are immersed in a common tunic. Such colonies, very diverse in shape, overgrow the surfaces of stones and underwater objects.
Most of all, single ascidians look like an oblong, swollen bag of irregular shape, growing with its lower part, which is called the sole, to various solid objects (Fig. 173, A). Two holes are clearly visible on the upper part of the animal, located either on small tubercles, or on rather long outgrowths of the body, resembling the neck of a bottle. These are siphons. One of them is oral, through which the ascidia sucks in water, the second is cloacal. The latter is usually somewhat shifted to the dorsal side. Siphons can be opened and closed with the help of muscles - sphincters. The body of the ascidian is dressed in a single-layer cell cover - the epithelium, which allocates on its surface a special thick shell - the tunic. The outer color of the tunic is different. Ascidians are usually colored in orange, reddish, brown-brown or purple tones. However, deep-sea ascidians, like many other deep-sea animals, lose their color and become off-white. Sometimes the tunic is translucent and through it the insides of the animal shine through. Often the tunic forms wrinkles and folds on the surface, overgrown with algae, hydroids, bryozoans and other sedentary animals. In many species, its surface is covered with grains of sand and small pebbles, so that the animal can be difficult to distinguish from surrounding objects.
Tunic is gelatinous, cartilaginous or jelly-like consistency. Its remarkable feature is that it consists of more than 60% cellulose. The thickness of the walls of the tunic can reach 2-3 cm, but usually it is much thinner.
Part of the cells of the epidermis can penetrate into the thickness of the tunic and populate it. This is possible only because of its gelatinous consistency. In no other group of animals do cells inhabit formations of a similar type (for example, the cuticle in nematodes). In addition, blood vessels can also grow into the thickness of the tunic.
Under the tunic lies the actual body wall, or mantle, which includes a single-layer ectodermal epithelium covering the body, and a connective tissue layer with muscle fibers. The outer muscles consist of longitudinal, and the inner of the annular fibers. Such muscles allow ascidians to make contractile movements and, if necessary, to throw water out of the body. The mantle covers the body under the tunic so that it lies freely inside the tunic and fuses with it only in the region of the siphons. In these places are sphincters - muscles that close the openings of the siphons.
There is no solid skeleton in the body of ascidians. Only some of them have small calcareous spicules of various shapes scattered in different parts of the body.
The digestive canal of ascidians begins with a mouth located at the free end of the body on the introductory, or oral, siphon (Fig. 173, B). Around the mouth is a corolla of tentacles, sometimes simple, sometimes quite strongly branched. The number and shape of the tentacles are different in different species, but there are never less than 6 of them. A huge pharynx hangs inward from the mouth, occupying almost the entire space inside the mantle. The pharynx of ascidians forms a complex respiratory apparatus. Gill slits, sometimes straight, sometimes curved, are located along its walls in a strict order in several vertical and horizontal rows (Fig. 173, B). Often the walls of the pharynx form 8-12 rather large folds hanging inward, located symmetrically on its two sides and greatly increasing its inner surface. The folds are also pierced by gill slits, and the slits themselves can take on very complex shapes, twisting in spirals on cone-shaped outgrowths on the walls of the pharynx and folds. The gill slits are covered with cells bearing long cilia. In the intervals between the rows of gill slits, blood vessels pass, also correctly located. Their number can reach 50 on each side of the pharynx. Here the blood is enriched with oxygen. Sometimes the thin walls of the pharynx contain small spicules to support them.
Gill slits, or stigmas, of sea squirts are invisible if you look at the animal from the outside, removing only the tunic. From the deep they lead to a special cavity lined with endoderm and consisting of two halves fused on the ventral side with the mantle. This cavity is called peribranchial, atrial or peribranchial (Fig. 173, B). It lies on each side between the pharynx and the outer wall of the body. Part of it forms a cloaca. This cavity is not an animal body cavity. It develops from special protrusions of the outer surface into the body. The peribranchial cavity communicates with the external environment through the cloacal siphon.
A thin dorsal plate hangs from the dorsal side of the pharynx, sometimes dissected into thin tongues, and a special sub-gill groove, or endostyle, runs along the ventral side. By beating the cilia on the stigmas, the ascidian drives water so that a direct current is established through the mouth opening. Further, water is driven through the gill slits into the peribranchial cavity and from there through the cloaca to the outside. Passing through the cracks, water releases oxygen into the blood, and various small organic residues, unicellular algae, etc. are captured by the endostyle and are driven along the bottom of the pharynx to its posterior end. Here is an opening leading to a short and narrow esophagus. Curving to the ventral side, the esophagus passes into a swollen stomach, from which the intestine emerges. The intestine, bending, forms a double loop and opens with an anus into the cloaca. Excrement is pushed out of the body through the cloacal siphon. Thus, the digestive system of ascidians is very simple, but the presence of an endostyle, which is part of their hunting apparatus, attracts attention. Endostyle cells of two genera - glandular and ciliated. The ciliated cells of the endostyle trap food particles and drive them to the pharynx, gluing them together with secretions of glandular cells. It turns out that the endostyle is a homologue of the thyroid gland of vertebrates and secretes an organic substance containing iodine. Apparently, this substance is close in composition to the thyroid hormone. Some ascidians have special folded outgrowths and lobed masses at the base of the walls of the stomach. This is the so-called liver. It is connected to the stomach by a special duct.
The circulatory system of ascidia is not closed. The heart is located on the ventral side of the animal's body. It looks like a small elongated tube surrounded by a thin pericardial sac, or pericardium. From two opposite ends of the heart departs along a large blood vessel. From the anterior end, the gill artery begins, which stretches in the middle of the ventral side and sends numerous branches from itself to the gill slits, giving small side branches between them and surrounding the gill sac with a whole network of longitudinal and transverse blood vessels. The intestinal artery departs from the posterior dorsal side of the heart, giving branches to the internal organs. Here, blood vessels form wide gaps, spaces between organs that do not have their own walls, very similar in structure to the gaps in bivalve mollusks. Blood vessels also go into the wall of the body and even into the tunic. The entire system of blood vessels and lacunae opens into the gill-intestinal sinus, sometimes called the dorsal vessel, to which the dorsal ends of the transverse gill vessels are also connected. This sinus is significant in size and stretches in the middle of the dorsal part of the pharynx. All tunicates, including sea squirts, are characterized by a periodic change in the direction of blood flow, since their heart alternately contracts for some time, either from back to front, then from front to back. When the heart contracts from the dorsal region to the abdominal region, the blood moves through the branchial artery to the pharynx, or gill sac, where it is oxidized and from there enters the enterobranchial sinus. The blood is then pushed into the intestinal vessels and back to the heart, just as it is in all vertebrates. With the subsequent contraction of the heart, the direction of the blood flow is reversed, and it flows, as in most invertebrates. Thus, the type of circulation in tunicates is transitional between the circulation of invertebrates and vertebrates. The blood of ascidians is colorless, sour. Its remarkable feature is the presence of vanadium, which takes part in the transport of oxygen by the blood and replaces iron.
The nervous system in adult ascidians is extremely simple and much less developed than in the larva. Simplification of the nervous system occurs due to the sedentary lifestyle of adult forms. The nervous system consists of the supraesophageal, or cerebral, ganglion, located on the dorsal side of the body between the siphons. From the ganglion, 2-5 pairs of nerves originate, going to the edges of the mouth opening, pharynx and to the insides - the intestines, genitals and to the heart, where there is a nerve plexus. Between the ganglion and the dorsal wall of the pharynx there is a small paranervous gland, the duct of which flows into the pharynx at the bottom of the fossa in a special ciliated organ. This piece of iron is sometimes considered a homologue of the lower appendage of the brain of vertebrates - the pituitary gland. Sensory organs are absent, but probably the mouth tentacles have a tactile function. Nevertheless, the nervous system of the tunicates is not essentially primitive. Ascidian larvae have a spinal tube lying under the notochord and forming a swelling at its anterior end. This swelling, apparently, corresponds to the brain of vertebrates and contains larval sensory organs - pigmented eyes and an organ of balance, or statocysts. When the larva develops into an adult animal, the entire back of the neural tube disappears, and the cerebral vesicle, together with the larval sense organs, disintegrates; due to its dorsal wall, the dorsal ganglion of the adult ascidian is formed, and the abdominal wall of the bladder forms the paranervous gland. As V. N. Beklemishev notes, the structure of the nervous system of tunicates is one of the best evidence of their origin from highly organized mobile animals. The nervous system of ascidian larvae is higher in development than the nervous system of the lancelet, which lacks a brain bladder.
Ascidians have no special excretory organs. Probably, the walls of the alimentary canal take part in the excretion to some extent. However, many ascidians have special so-called scattered accumulation buds, consisting of special cells - nephrocytes, in which excretion products accumulate. These cells are arranged in a characteristic pattern, often clustered around the intestinal loop or gonads. The reddish-brown color of many ascidians depends precisely on the excretions accumulated in the cells. Only after the death of the animal and the decay of the body, the waste products are released and go into the water. Sometimes in the second knee of the intestine there is an accumulation of transparent vesicles that do not have excretory ducts, in which concretions containing uric acid accumulate. In representatives of the Molgulidae family, the accumulation bud becomes more complicated and the accumulation of vesicles turns into one large isolated sac, the cavity of which contains concretions. The great originality of this organ lies in the fact that the kidney sac of molgulids in some other ascidians always contains symbiotic fungi that do not even have distant relatives among other groups of lower fungi. Fungi form the thinnest filaments of micelles, braiding concretions. Among them there are thicker formations of irregular shape, sometimes sporangia with spores are formed. These lower fungi feed on urates, the products of ascidian excretion, and their development frees the latter from accumulated excretions. Apparently, these fungi are necessary for ascidia, since even the rhythm of reproduction in some forms of ascidia is associated with the accumulation of excreta in the kidneys and with the development of symbiotic fungi. How fungi are transferred from one individual to another is unknown. Ascidian eggs are sterile in this respect, and young larvae do not contain fungi in the kidney, even when the excretions are already accumulating in them. Apparently, young animals are again "infected" with fungi from sea water.
Ascidians are hermaphrodites, that is, the same individual has both male and female gonads at the same time. The ovaries and testes lie one or several pairs on each side of the body, usually in a loop of intestine. Their ducts open into the cloaca, so that the cloacal opening serves not only for the exit of water and excrement, but also for the excretion of sexual products. Self-fertilization does not occur in ascidians, since eggs and sperm mature at different times. Fertilization most often occurs in the peribranchial cavity, where the spermatozoa of another individual penetrate with a current of water. Rarely is it outside. Fertilized eggs exit through the cloacal siphon, but sometimes the eggs develop in the peribranchial cavity and already formed floating larvae emerge. Such a live birth is especially characteristic of colonial ascidians.
In addition to sexual reproduction, ascidia also reproduce asexually by budding. In this case, various ascidian colonies are formed. The structure of an ascidiozooid - a member of a colony of complex ascidians - in principle does not differ from the structure of a single form. But their dimensions are much smaller and usually do not exceed a few millimeters. The body of the ascidiozooid is elongated and divided into two or three sections (Fig. 174, A): the pharynx is located in the first, thoracic, section, the intestines are in the second, and the gonads and heart are in the third. Sometimes different organs are located somewhat differently.
The degree of communication between individual individuals in the ascidiozooid colony may be different. Sometimes they are completely independent and are connected only by a thin stolon that spreads along the ground. In other cases, ascidiozooids are enclosed in a common tunic. They can either be scattered in it, and then both oral and cloacal openings of ascidiozooids come out, or they are arranged in regular figures in the form of rings or ellipses (Fig. 174, B). In the latter case, the colony consists of groups of individuals with independent mouths, but having a common cloacal cavity with one common cloacal opening, into which the cloacae of individual individuals open. As already mentioned, the dimensions of such ascidiozooids are only a few millimeters. In the case when the connection between them is carried out only with the help of a stolon, ascidiozooids reach larger sizes, but usually smaller than single ascidians.
The development of ascidians, their asexual and sexual reproduction will be described below.
Pyros structure.
Pyrosomes, or fireballs, are free-floating colonial pelagic tunicates. They got their name because of the ability to glow with bright phosphorescent light.
Of all the planktonic forms of tunicates, they are closest to the sea squirts. Essentially, these are colonial sea squirts floating in the water. Each colony consists of many hundreds of individual individuals - ascidiozooids, enclosed in a common, often very dense tunic (Fig. 175, A). In pyrosomes, all zooids are equal and independent in terms of nutrition and reproduction. The colony is formed by budding of individual individuals, and the kidneys fall into their place, moving in the thickness of the tunic with the help of special wandering cells - phorocytes. The colony has the shape of a long, elongated cylinder with a pointed end, which has a cavity inside and is open at its wide rear end (Fig. 175, B). Outside, the pyrosome is covered with small, soft, spiny outgrowths. Their most important difference from the colonies of sessile ascidians lies also in the strict geometric regularity of the shape of the colony. Individual zooids stand perpendicular to the wall of the cone. Their mouth openings are turned outward, and the cloacal openings are on the opposite side of the body and open into the cavity of the cone. Separate small ascidiozooids capture water with their mouths, which, having passed through their body, enters the cavity of the cone. The movements of individual individuals are coordinated among themselves, and this coordination of movements occurs mechanically in the absence of muscle, vascular or nerve connections. In the tunic, mechanical fibers are stretched from one individual to another by pyros, connecting their motor muscles. The contraction of the muscle of one individual pulls the other individual with the help of the fibers of the tunic and transmits irritation to it. Contracting simultaneously, small zooids push water through the cavity of the colony. In this case, the entire colony, similar in shape to a rocket, having received a reverse push, moves forward. Thus, pyrosomes have chosen for themselves the principle of jet propulsion. This method of movement is used not only by pyrosomes, but also by other pelagic tunicates.
The pyrosom tunic contains such a large amount of water (in some tunicates, water is 99% of body weight) that the entire colony becomes transparent, as if glass, and almost invisible in the water. However, there are also pink-colored colonies. Such gigantic pyrosomes - their length reaches 2, 5 and even 4 m, and the diameter of the colony is 20-30 cm - have been repeatedly caught in the Indian Ocean. Their name is Pyrosoma spinosum. The tunic of these pyrosomes has such a delicate consistency that, getting into plankton nets, the colonies usually break up into separate pieces. Usually, the dimensions of pyrosoma are much smaller - from 3 to 10 cm long with a diameter of one to several centimeters. A new species of pyrosomes, P. vitjasi, has recently been described. The colony of this species also has a cylindrical shape and sizes up to 47 cm. According to the author's description, through the pinkish mantle, as dark brown (or rather, dark pink in living specimens) inclusions, the insides of individual ascidiozooids shine through. The mantle has a semi-liquid consistency, and if the surface layer is damaged, its substance spreads in water in the form of viscous mucus, and individual zooids freely disintegrate.
The structure of the ascidiozooid pyrosome is not much different from the structure of a single ascidian, except that its siphons are located on opposite sides of the body, and are not brought together on the dorsal side (Fig. 175, B). The sizes of ascidiozooids are usually 3-4 mm, and in giant pyrosomes, up to 18 mm in length. Their body may be laterally flattened or oval. The mouth opening is surrounded by a corolla of tentacles, or only one tentacle may be present on the ventral side of the body. Often the mantle in front of the mouth opening, also on the ventral side, forms a small tubercle or a rather significant outgrowth. The mouth is followed by a large pharynx, cut through by gill slits, the number of which can reach 50. These slits are located either along or across the pharynx. Approximately perpendicular to the gill slits are blood vessels, the number of which also varies from one to three to four dozen. The pharynx has an endostyle and dorsal tongues hanging down into its cavity. In addition, in the anterior part of the pharynx, on the sides, there are luminous organs, which are accumulations of cell masses. In some species, the cloacal siphon also has luminous organs. The luminous organs of pyrosomes are inhabited by symbiotic luminous bacteria. Under the pharynx lies a nerve ganglion, there is also a paranervous gland, the canal of which opens into the pharynx. The muscular system of ascidiozooids pyrosomes is poorly developed. There are fairly well-defined circular muscles located around the oral siphon, and an open ring of muscles near the cloacal siphon. Small bundles of muscles - dorsal and abdominal - are located in the corresponding places of the pharynx and radiate along the sides of the body. In addition, there are also a couple of cloacal muscles. Between the dorsal part of the pharynx and the body wall there are two hematopoietic organs, which are oblong clusters of cells. Propagating by division, these cells turn into various elements of the blood - lymphocytes, amoebocytes, etc.
The digestive section of the intestine consists of the esophagus extending from the back of the pharynx, stomach and intestines. The intestine forms a loop and opens with an anus into the cloaca. On the ventral side of the body lies the heart, which is a thin-walled sac. There are testes and ovaries, the ducts of which also open into the cloaca, which can be more or less extended and opens with a cloacal siphon into the common cavity of the colony. In the region of the heart, ascidiozooids pyrosomes have a small finger-like appendage - the stolon. It plays an important role in colony formation. As a result of the division of the stolon in the process of asexual reproduction, new individuals bud from it.
Salp structure.
Like pyrosomes, salps are free-swimming animals and lead a pelagic lifestyle. They are divided into two groups: kegs, or doliolid(Cyclomyaria), and salp proper(Desmomyaria). These are completely transparent animals in the form of a barrel or cucumber, at the opposite ends of which there are mouth and anus openings - siphons. Only in some species of salps, certain parts of the body, such as the stolon and intestines, are painted in living specimens in a bluish-blue color. Their body is dressed in a delicate transparent tunic, sometimes equipped with outgrowths of different lengths. A small, usually greenish-brown intestine is well visible through the walls of the body. Salps range in size from a few millimeters to several centimeters in length. The largest salpa - Thetys vagina - was caught in the Pacific Ocean. The length of her body (together with appendages) was 33.3 cm.
The same types of salps are found either in single forms or in the form of long chain-like colonies. Such chains of salps are separate individuals connected to each other in a row. The connection between zooids in a salp colony, both anatomically and physiologically, is extremely weak. The members of the chain, as it were, stick together with each other with attachment papillae, and in essence their coloniality and dependence on each other are barely expressed. Such chains can reach lengths of more than one meter, but they are easily torn apart, sometimes simply by the impact of a wave. Individuals and individuals that are members of the chain differ so much from each other both in size and in appearance that they were even described by old authors under different species names.
Representatives of another order - kegs, or doliolids - on the contrary, build extremely complex colonies. One of the greatest contemporary zoologists, V.N. Beklemishev, called barrel owls one of the most fantastic creatures in the sea. Unlike ascidians, in which the formation of colonies occurs due to budding, the emergence of colonies in all salps is strictly related to the alternation of generations. Solitary salps are nothing more than asexual individuals that have emerged from eggs, which, budding, give rise to the colonial generation.
As already mentioned, the body of an individual, whether it is either a single individual or a member of a colony, is dressed in a thin transparent tunic. Under the tunic, like the hoops of a barrel, whitish ribbons of circular muscles shine through. They have 8 such rings. They encircle the body of the animal at a certain distance from each other. In kegs, the muscle bands form closed hoops, while in the salps proper, they do not close on the ventral side. Consistently contracting, the muscles push the water entering through the mouth through the body of the animal and push it out through the excretory siphon. Like
Our distant relatives - tunicates
From the book Escape from Loneliness author Panov Evgeny NikolaevichOur distant relatives - tunicates The third large group of attached marine animals, which at one time were also classified as zoophytes, are ascidians. Scientists have described about 1 thousand species of ascidians, many of which exist in the form of colonies. "Thickets" of ascidians are much