Class Crustacea. Higher and lower crayfish are intermediate hosts of human helminths. Crustaceans
Subclass Gillpods
The most primitive These small crustaceans have leaf-shaped legs and are used equally for locomotion and respiration. They also create a current of water that brings food particles to the mouth. Their eggs easily tolerate desiccation and wait in the soil for the new rainy season. Artemia is interesting among the branchiopods: it can live in salt lakes with a salt concentration of up to 300 g / l, and dies in fresh water after 2-3 days.
1) breathing with gills;
2) fusion of the head and thoracic regions with the formation of the cephalothorax;
3) the presence of two pairs of antennae that perform tactile and olfactory functions, a pair of complex or faceted eyes, and three pairs of mouth limbs (a pair of upper and two pairs of lower jaws that capture and grind food);
4) a diverse structure of the thoracic limbs, which perform the functions of holding and moving food to the mouth, moving the body, breathing;
5) the abdominal limbs are used for swimming, and in females for attaching fertilized eggs;
6) crustaceans of all age groups molt, but juveniles more often than adults.
Features of the structure and processes of life. Crayfish is a characteristic representative of the Crustacea class. Lives in fresh low-flowing water bodies. Active at dusk and at night. Crayfish are omnivorous: they eat plant foods, live and dead prey. Reaching a significant size (15 cm or more) and having good taste, crayfish is a valuable commercial object.
The body of crayfish consists of 18 segments, united in the cephalothorax and abdomen. It is covered with a thick layer of chitinous cuticle, reinforced with lime deposits. The uppermost wax-like layer of the cuticle, which prevents the evaporation of water from the body in terrestrial arthropods, is absent in crustaceans, which explains their existence exclusively in an aquatic or near-aquatic environment.
The head consists of a head lobe bearing a pair of antennae - antennules (first antennae), and four segments, each of which has paired transformed limbs: antennae (second antennae), upper jaws, and first and second lower jaws. The thoracic region is formed by eight segments, bearing three pairs of mandibles and five pairs of walking limbs. The jointed mobile abdomen has six segments, each of which has a pair of swimming limbs. In males, the first and second pair of abdominal limbs are long, groove-like and are used as a copulatory organ. In the female, the first pair of limbs is greatly shortened. The abdomen ends with a caudal fin formed by the sixth pair of wide lamellar limbs and a caudal lobe.
Gills in crayfish are thin-walled feathery outgrowths of the skin of the thoracic limbs and the side walls of the thoracic part of the body. They are located on the sides of the chest in the gill cavity, covered by the cephalothorax. The circulation of water in the gill cavity is provided by the movement of a special process of the second pair of lower jaws (200 times per minute).
Digestive system begins with a mouth opening located on the underside of the head. Through it, food crushed by the mouth limbs passes through a short pharynx and esophagus into the stomach, which consists of two sections - chewing and filtering. On the inner walls of the chewing part of the stomach are chitinous teeth, with the help of which the food is ground. The food slurry is filtered through the bristles of the filter section, and its liquid part enters the midgut and digestive gland ("liver"), where it is digested and absorbed. The hindgut in the form of a straight tube is located in the abdomen of the crayfish and opens with an anus at its end.
Circulatory system typical of all arthropods - open with a compact heart in the form of a pentagonal sac on the dorsal side of the cephalothorax.
Metabolic products are removed through the excretory organs - paired green glands lying at the base of the head and opening outward at the base of the antennae. In their structure, the glands resemble modified metanephridia, which carry metabolic products out of the body cavity.
Cancer eyes are complex. They consist of a large number of individual eyes, or facets, separated from each other by thin layers of pigment. Vision is mosaic, since each facet sees only part of the object. The eyes are located on movable stalks. The mobility of the eye compensates for the immobility of the head. The organs of touch are long whiskers - antennas, and the organs of smell - short whiskers - antennules. At the base of the short whiskers is the organ of balance.
At the end of winter, females lay fertilized eggs on their abdominal limbs. At the beginning of summer, rachata hatch from eggs, which are protected by the female for a long time, hiding on her abdomen from the underside. Young crayfish grow intensively and molt several times a year; adults molt only once a year. Then soft chitin is formed in the cancer. After some time, it is impregnated with lime, hardens and cancer growth stops until the next molt.
The role of crustaceans in nature and their practical significance. Crustaceans are of great importance in nature and human economy. Countless crustaceans inhabiting marine and fresh waters serve as food for many species of fish, cetaceans and other animals. Daphnia, cyclops, diaptomuses, bokogshavy - an excellent food for freshwater fish and their game. Many small crustaceans feed on the filtration method, i.e., they filter out the food suspension with their thoracic limbs. Thanks to their food activity, natural water is clarified and its quality is improved.
Many large crustaceans are commercial species, such as lobsters, crabs, spiny lobsters, shrimps, crayfish. Medium-sized marine crustaceans are used by humans to make a nutritious protein paste.
Crustaceans, or crayfish, evolved from trilobite arthropods that moved to faster movement at the bottom of reservoirs and in the water column. Due to a more active lifestyle, the organization of crustaceans has become much more complicated compared to their ancestors. This is a large and diverse class, whose representatives live in marine, fresh and brackish waters. Only a few crustaceans live on land, but only in humid places.
Outdoor building. The structure of crayfish (see Fig. 75, 80) is very diverse. The division of the body into sections in different groups is not similar. Often the head and thoracic regions fuse together to form the cephalothorax, to which the jointed abdomen is connected. The size of the body varies widely: many forms are microscopic organisms that live mainly in the water column; bottom forms often reach large sizes. The cuticle of crustaceans, like that of all aquatic arthropods, consists of two main layers: the inner one - the endocuticle, and the outer one - the exocuticle (Fig. 78). The latter is impregnated with tannins and therefore very durable. During molting, the endocuticle dissolves and is absorbed by the hypodermis, while the exocuticle is insoluble and is shed entirely. Large crayfish are covered with strong shells. Smaller forms may also have shell formations, but for the most part the chitinous cuticle covering them is thin. In one order of lower crayfish (shell crustaceans), the body is enclosed in a bivalve calcareous shell. All crustaceans have two pairs of antennae, or antennae (Fig. 73, 80), whose structure and functions are not similar in different groups of the class (see below).
Nervous system. In a number of lower forms, the central section of this system consists of a relatively simple brain and abdominal cords, which form a ladder, and not a chain (see Fig. 72), in other crustaceans, the brain becomes more complex (to varying degrees in different groups), abdominal cords form a chain, the nodes of which, as the concentration of the body increases, can be connected up to the merging of all nodes into one (see Fig. 72). The behavior of the higher representatives of the class, which, as a rule, are active predators reaching very large sizes, is highly complicated and is ensured by progressive changes in the entire nervous system. The organs of touch in the form of sensitive bristles are scattered all over the body, but there are especially many of them on the antennae. The organs perceiving chemical irritations are quite well developed; in large crayfish, they are concentrated mainly on the antennae of the first pair. Balance organs (statocysts) are distributed mainly in higher crayfish and are located in the first segment of the first pair of antennae (Fig. 79).
Eyes can be simple or complex. Compound, or faceted, eyes (Fig. 79) consist of a large number of individual eyes, or ommatidia. Each ommatidium consists of a cornea (the transparent part of the chitinous cuticle), a crystal cone - an elongated transparent body, which is adjacent to nerve, or retinal, cells that secrete light-sensitive rods (rhabdoms) on their inner edges. Ommatidia are separated from each other by pigment cells. Rays falling obliquely on the ommatidia are absorbed by the pigment cells, which isolate the ommatidia from each other, and do not reach the nerve cells. The latter perceive only those rays that fall perpendicular to the surface of the ommatidium. Thus, each ommatidium perceives only a part of the object, yet the ommatidium perceives the entire object. The image of an object in a compound eye is made up of its individual parts and resembles mosaic pictures (or mosaics) made up of multi-colored pebbles or plates. Therefore, such vision is called mosaic. Many large crayfish have compound eyes located on special stalks.
Propulsion system. The movement of crayfish is accomplished with the help of different limbs - antennae or legs in planktonic, usually small forms (Fig. 80), special walking legs in benthic, usually large forms (see Fig. 73). In addition, the latter can swim, thanks to the strong bending of the abdomen under the chest. In crayfish, unlike terrestrial arthropods, biramous limbs are widespread, which, together with setae, have a wide surface and are convenient for using them as oars. In large crayfish, for example, in the river, the branches of the hind pair of legs have turned into two wide plates (see Fig. 73), which, together with the last, very wide segment of the abdomen, help well in scooping up water with the abdomen.
Circulatory system. The heart, like all arthropods, located on the dorsal side, is present in most crustaceans (see Fig. 75, 80, A). The shape of the heart varies from a long tube to a compact pouch. In a number of small forms, the heart is absent and the movement of blood is caused in them by movements of the intestines, as well as movements of the whole body. The development of a network of blood vessels mainly depends on the size of the body: in large crayfish, it can be developed quite well, in small ones it can be completely reduced.
Respiratory system. The respiratory organs in most crustaceans are gills, which are leg appendages that have different shapes: in small crayfish, these are rounded leaves (Fig. 80, A), in large crayfish (such as crayfish) they are finely dissected (see. Fig. 75), due to which their surface increases. The change of water near the gills occurs due to the movement of the legs on which they are located, as well as due to the movement of certain limbs that do not have gills. A fairly significant number of small species do not have gills, and they absorb oxygen through the surface of the body, mainly in its thinner parts.
excretory system. The excretory system is represented mainly by a pair, rarely more, of metanephridia. The decrease in the number of these organs in comparison with annelids, in which they are numerous, is mainly due to the fact that in crustaceans the body cavity is continuous, not divided by septa, as in rings, and it is enough for them to have a small number of excretory organs, but more complexly arranged, separated into a number of departments (Fig. 81). In higher cancers, the metanephridia reach especially great complexity, they are large (about 1 cm or more) and open at the base of the antennas of the second pair and are therefore called antennal. In other cancers, metanephridia are simpler, they are smaller (see Fig. 80, A) and open at the base of the second pair of lower jaws, or maxilla, which is why they got the name maxillary.
Digestive system. The digestive system is very diverse. Small crustaceans (see Fig. 80), living in the water column, receive food (organic pieces, bacteria, algae, microscopic animals) as a result of vigorous work in some - antennae, in others - mouth limbs, in others - pectoral legs, creating continuous flow of water. In the daphnia crustacean, the hind pectoral legs beat 200-300 times per minute and ensure that food enters the mouth. Large crayfish (see Fig. 73) capture prey with the help of legs armed with claws.
Crustaceans, like all arthropods, have limbs that surround the mouth and perform a number of functions. The mouth limbs of river and other crayfish, for example, include (see Fig. 73) well-developed mandibles, or upper jaws, with a jointed palp and a plate, the inner edge of which is serrated and serves to grind food, and two pairs of lower jaws, which also serve for the mechanical processing of food. In addition, three pairs of mandibles, located already on the chest, help hold food and forward it to the mouth. In the anterior part of the digestive apparatus, many species develop a large chewing stomach (see Fig. 75), the walls of which are thickened due to cuticular formations and serve for the mechanical processing of food. Digestion of food occurs in the midgut, into which the ducts of the digestive gland, called the liver, flow. In fact, this gland performs the functions of the pancreas and hepatic glands of vertebrates, since it secretes juice that promotes the digestion of all major organic compounds - proteins, carbohydrates and fats: the liver of vertebrates plays a large role mainly in the digestion of fats. Therefore, the digestive gland of crayfish is more correctly called pancreas-hepatic. In small crustaceans, these glands are moderately developed, in the form of hepatic processes (see Fig. 80, A, 10), in large crayfish it is a large organ, consisting of several lobes (see Fig. 75).
Reproduction. Reproduction is sexual. Most species are dioecious. Males, as a rule, differ greatly from females in body size, limb structure, etc. Parthenogenesis is widespread in some groups of lower crayfish. In branched crustaceans, which include many species (for example, various daphnia) that serve as food for fish, most of the warm season there are only females that lay unfertilized eggs, from which new crustaceans quickly develop. Males usually appear before the onset of the cold season or other adverse conditions. Females fertilized by males lay eggs surrounded by strong, thick shells that do not develop until the following year. Many crayfish hatch eggs on the abdomen or in a special brood chamber (see Fig. 80, A).
Development. Development with transformation or direct. In lower crustaceans, developing with transformation, larvae emerge from eggs, called nauplii(Fig. 82). These larvae have three pairs of legs and one eye. In higher crayfish living in the sea, larvae, called zoea, mostly emerge from eggs (Fig. 82). Zoeas have more limbs than nauplii and two compound eyes; they are studded with spikes that increase their surface and facilitate soaring in the water. Other species of larvae are also known, which occupy an intermediate position between the nauplius and the zoea, or between the zoea and the adult form. In many lower freshwater crustaceans and crayfish, development is direct.
Crayfish growth is always associated with molting; for example, crayfish molt 10 times during the first year of its life and therefore grows rapidly (from 0.9 to 4.5 cm), during the second year it molts 5 times, during the third - only twice, and then the females molt once a year, and males - 2 times. After 5 years, they almost do not grow; live 15 - 20 years.
Origin. Crustaceans originated, as noted above, from arthropods close to trilobites. In connection with their adaptation to a more active and complex way of life, the differentiation of the body into sections increased, many segments merged, i.e., the concentration of the organism increased; the nervous system has become more complicated; the structure of the limbs (generally the same in trilobites) became diverse due to the performance of various functions; the intensity of the work of other organ systems has increased.
lower crustaceans
Subclass Gillpods
The most primitive These small crustaceans have leaf-shaped legs and are used equally for locomotion and respiration. They also create a current of water that brings food particles to the mouth. Their eggs easily tolerate desiccation and wait in the soil for the new rainy season. Artemia is interesting among the branchiopods: it can live in salt lakes with a salt concentration of up to 300 g / l, and dies in fresh water after 2-3 days.
Subclass Maxillopods (maxillopods)
Representatives of the order of barnacles are amazing: sea acorns and sea ducks. These sea crayfish have moved to a sedentary lifestyle in houses made of calcareous plates. The larva is a typical nauplius, sinks to the bottom and is attached by antennules. The antennae and the entire anterior part of the head turn into an attachment organ (a long fleshy stalk in sea ducks, or a flat wide sole in sea acorns), antennae and compound eyes atrophy, pectoral legs stretch into long two-branched "antennae" that drive food to the mouth.