Lower cancers. Class Crustaceans. Higher and lower crayfish - intermediate hosts of human helminths What is the difference between higher crayfish and lower ones

Description

The body of crustaceans is divided into the following sections: head, thoracic and abdominal. In some species, the head and thorax are fused together (cephalothorax). Crustaceans have an external skeleton (exoskeleton). The cuticle (outer layer) is often reinforced with calcium carbonate, which provides additional structural support (especially true for large species).

Many species of crustaceans have five pairs of appendages on their heads (these include: two pairs of antennae (antennae), a pair of lower jaws (maxillae), and a pair of upper jaws (mandibles, or mandibles)). The compound eyes are located at the end of the stalks. The thorax contains several pairs of pereiopods (walking legs), and the segmented belly contains pleopods (abdominal legs). The posterior end of the crustacean body is called the telson. Large species of crustaceans breathe with gills. Small species for gas exchange using the surface of the body.

reproduction

Most species of crustaceans are heterosexual and reproduce sexually, although some groups, such as barnacles, remipedians, and cephalocarids, are hermaphrodites. The crustacean life cycle begins with a fertilized egg, which is either released directly into the water or attached to the genitals or legs of the female. After hatching from an egg, crustaceans go through several stages of development before turning into an adult.

food chain

Crustaceans occupy a key place in the sea and are one of the most common animals on Earth. They feed on organisms such as phytoplankton, in turn, crustaceans become food for larger animals such as fish, and some crustaceans such as crabs, lobsters and shrimp are a very popular food for humans.

Dimensions

Crustaceans come in a wide variety of sizes from microscopic aquatic fleas and crustaceans to the giant Japanese spider crab, which reaches a mass of about 20 kg and has legs 3-4 m long.

Food

In the process of evolution, crustaceans have acquired a wide range of feeding habits. Some species are filter feeders, extracting plankton from the water. Other species, especially large ones, are active predators that grab and tear their prey with powerful appendages. There are also scavengers, especially among small species, feeding on the decaying remains of other organisms.

First crustaceans

Crustaceans are well represented in the fossil record. The first representatives of crustaceans belong to the Cambrian period and are represented by fossils mined in the Burges Shale Shale Formation, located in Canada.

Classification

Crustaceans include the following 6 classes:

• Gillnopods (Branchiopoda);
• Cephalocarids (Cephalocarida);
• higher crayfish (Malacostraca);
• Maxillopods (Maxillopoda);
• Shellfish (Ostracoda);
• crested (remipedia).

The most primitive crustaceans belong to the subclass Gillnopods(Branchiopoda). Daphnia(Daphnia) are representatives of the Leaf-legged order, the branched mustache suborder. Daphnia, inhabitants of the water column, are often called water fleas, probably because of their small size and hopping mode of movement. Let's put some living D. magna in a glass jar and observe them. The body of the crustaceans is up to 6 mm long, covered with a bivalve shell flattened laterally. A large black spot stands out on a small head - an eye, and a brownish-greenish intestine clogged with food shines through in the trunk region.

Daphnia never rest for a second. The main role in the movement is played by the flapping of long lateral antennae. The legs of Daphnia are leaf-shaped, small, they do not take any part in the movement, but they regularly serve for nutrition and respiration. The legs are constantly working, making up to 500 strokes per minute. So they create a current of water that carries algae, bacteria, yeast and oxygen.

The cladocerans also include such pelagic crustaceans as a small (less than 1 mm in length) bosmina longnosed(Bosmina longirostris). It is easily recognizable by its long, curved nose - the rostrum - with a tuft of bristles in the middle. An even smaller owner of a brownish spherical shell - hydrorus spherical(Chydorus sphaericus) - can be found in the water column and among coastal thickets.

Also widespread copepods(Copepoda) - Cyclops and Diaptomus, which belong to the subclass Maxillopod(Maxillopoda). Their body consists of a head, segmented chest and abdomen. The main organ of movement is powerful antennae and pectoral legs bearing swimming bristles. The legs work synchronously, like oars. Hence the common name of crustaceans - "copepods".

Diaptomuses, like Daphnia, are quite peaceful animals. In a glass vessel, you can easily observe their movement. Diaptomuses(Eudiaptomus graciloides) soar smoothly, balancing with outstretched antennae, the length of which is almost equal to the length of the entire body. Having gone down, they make a sharp stroke with their pectoral legs and short abdomen and "jump" up. The current of water, carrying food, is created by crustaceans with short second antennae, making several hundred beats per minute. The elongated body of the crustacean is translucent and colorless, they need to be invisible to predators. Diaptomus females often carry a small pouch filled with eggs under their belly. Males are easily distinguished by the right antenna with a knot in the middle and the last pair of legs, which is complexly arranged, with long hooked outgrowths. These devices are used by the male to hold the female.

More common in fresh waters cyclops, named after the one-eyed hero of ancient Greek myths. There is only one eye on the head of these crustaceans! Cyclops (Cyclops kolensis) have short antennae; adult females carry their eggs in two bags on the sides of the abdomen. Males hold their partners with both anterior loop-shaped antennae. Cyclopes differ in fussy, seemingly erratic movement. They "jump" often and sometimes somersault in the water. The fast and chaotic movement of the Cyclopes is aimed at achieving two main goals: firstly, not to get caught in the mouth of a fish, and secondly, to have time to grab something edible. Cyclopes are by no means vegetarians. If a large algae comes across, they will also eat it, but they still prefer the juveniles of their cladocerous and copepod neighbors and other aquatic trifles, for example, ciliates and rotifers.

Crustaceans- These are aquatic arthropods or inhabitants of wet places. Their body sizes vary from a few millimeters to 1 m. They are ubiquitous; lead a free or attached lifestyle. The class includes about 20 thousand species. Only crustaceans are characterized by the presence of two pairs of antennae, biramous limbs, and gill breathing. The class Crustacea combines 5 subclasses. Conventionally, all representatives are divided into lower (daphnia, cyclops) and higher crayfish (lobster, spiny lobster, shrimp, crayfish).

Representative of higher cancers - river crayfish. It lives in fresh water bodies with running water, is nocturnal and is a predator.

Crayfish. External and internal structure:
1 - Antennae, 2 - Claw, 3 - Walking legs, 4 - Caudal fin, 5 - Abdomen, 6 - Cephalothorax, 7 - Head ganglion, 8 - Digestive tube, 9 - Green gland, 10 - Gills, 11 - Heart, 12 - gonad

The body of the cancer is covered with a dense chitinous shell. The fused segments of the head and chest form the cephalothorax. Its front part is elongated and ends with a sharp spike. Two pairs of antennae are located in front of the spine, and two complex (faceted) eyes are located on the sides on movable stalks. Each eye contains up to 3 thousand small eyes. Modified limbs (6 pairs) form the oral apparatus: the first pair is the upper jaws, the second and third are the lower jaws, the next three pairs are the jaws. The thoracic region bears 5 pairs of jointed limbs. The first pair is the organ of attack and defense. It ends with powerful pincers. The remaining 4 pairs are walking limbs. The limbs of the jointed abdomen are used in females for carrying eggs and cubs. The abdomen ends with a caudal fin. When the crayfish swims, it scoops up water with it and moves with its tail end forward. Bundles of striated muscles are attached to the internal protrusions of the chitinous cover.

Cancer feeds on both living organisms and decaying animal and plant debris. The crushed food enters through the mouth into the pharynx and esophagus, then into the stomach, which has two sections. Chitinous teeth of the chewing section grind food; in the filter stomach, it is filtered and enters the middle intestine. The ducts of a large digestive gland, which performs the functions of the liver and pancreas, also open here. Under the action of its secret, the food slurry is digested. Nutrients are absorbed, and undigested residues through the hindgut and anus are thrown out.

The excretory organs of cancer are a pair of green glands (modified metanephridia) that open at the base of long antennae. Respiratory organs - gills located on the sides of the cephalothorax. They are permeated with blood vessels in which gas exchange occurs - the blood gives off carbon dioxide and is saturated with oxygen. The circulatory system is not closed. It consists of a pentagonal heart located on the dorsal side and vessels extending from it. The blood pigment contains copper, which is why it is blue in color. The nervous system of crayfish resembles the nervous system of annelids. It consists of the supraglottic and subpharyngeal ganglia, united in the circumpharyngeal ring, and the ventral nerve cord. The organs of vision, touch and smell (on the antennae), balance (at the base of the short antennae) are well developed. Cancers are segregated. Reproduction is sexual, development is direct. Eggs are laid in winter; small crayfish hatch from eggs in early summer. Cancer expresses concern for offspring.

Significance of crustaceans. Crustaceans serve as food for aquatic animals and for humans (lobsters, crabs, shrimps, crayfish). They clean the water bodies from carrion. Some representatives of crustaceans cause diseases of fish, settling on their skin or gills, some are intermediate hosts for tapeworms and roundworms.

Class Crustaceans (Crustacea)

The class Crustacea (Crustacea) includes very diverse arthropods. This includes animals that are often not similar to each other in appearance and lifestyle, such as crabs and wood lice, crayfish and shrimps, hermit crabs and carp lice, lobsters and water fleas ... And since adult crustaceans are very diverse in form, it is almost impossible to give them a brief description that clearly distinguishes them from other groups of animals. Therefore, evolutionary (genetic) family ties between different representatives of the class are established only by the features of their larval development. And he, in turn, usually includes a complex metamorphosis, in which only the first larval stage - the nauplius - is common to all crustaceans. But some others, and in some cases all, including the first, may be absent, and then a copy of an adult animal immediately hatches from a fertilized egg, but only a miniature one ...

Some edible and harmful species of crustaceans have been known to man since ancient times, but most representatives of this class are known only to narrow specialists. As it turned out, crustaceans are among the most numerous on our planet. Currently, scientists have described more than 25,000 of their species. At the same time, most species of crustaceans live in the seas and oceans, therefore they are figuratively sometimes called "sea insects" for their abundance and diversity. However, many species of crustaceans also live in fresh waters and on land. Therefore, they can practically be found in all water bodies: under the ice in the polar regions, and in hot springs with temperatures up to 50 ° C, and in deserts, and at depths of up to 6 km, and the tops of tropical trees.

The economic importance of crustaceans is also great. At the same time, crabs, lobsters, crayfish and shrimps, which a person directly eats, are of great importance. But numerous small forms that swim en masse near the surface of water bodies as part of zooplankton and are often barely visible to the naked eye, form the main link in a number of food chains. It is these tiny crustaceans that are the link between microscopic planktonic algae with fish, whales and other large game animals. Without small crustaceans, which turn plant cells into easily digestible animal food, the existence of most representatives of the aquatic fauna would become almost impossible.

Among the crustaceans, there are many species that are harmful to humans, which in one way or another cause damage to the economy of a person or his health. Thus, drilling forms of crustaceans, such as the wood-boring shrimp, make passages in wooden port facilities and other underwater structures. On the bottoms of ships, powerful fouling of sea acorns and sea ducks form, which interfere with navigation. Some types of crabs, crayfish and some other crustaceans are carriers of human diseases in the tropics (and in the Russian Far East), while other crustaceans, like wood lice and shields, often damage vegetation, in particular rice crops, or farmed marine species.