Class Crustaceans. Higher and lower crayfish are intermediate hosts of human helminths. Crustaceans (ass. f. d. Mordukhai-Boltovskaya) Higher and lower crustaceans
Subclass Gill-footed
The most primitive. These small crustaceans have leaf-shaped legs and are used equally for movement and breathing. They also create a current of water that carries food particles to the mouth. Their eggs easily tolerate drying out and wait in the soil for the new rainy season. Artemia is an interesting branchiopod: it can live in salt lakes with a salt concentration of up to 300 g/l, and in fresh water dies after 2-3 days.
1) breathing using gills;
2) fusion of the head and thoracic regions to form the cephalothorax;
3) the presence of two pairs of antennae performing tactile and olfactory functions, a pair of compound, or facet, eyes, and three pairs of oral limbs (a pair of upper and two pairs mandibles that capture and crush food);
4) the varied structure of the thoracic limbs, which perform the functions of holding and moving food to the mouth, body movement, and 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 molt more often than adults.
Features of the structure and life processes. Crayfish is a characteristic representative of the class Crustaceans. Lives in fresh, low-flowing water bodies. Active during twilight and night time. Crayfish are omnivores: they eat plant foods, live and dead prey. Reaching significant sizes (15 cm or more) and having good taste qualities, crayfish is a valuable commercial object.
The body of the crayfish consists of 18 segments, combined into the cephalothorax and abdomen. It is covered with a thick layer of chitinous cuticle, strengthened by 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 semi-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 jaws and five pairs of walking limbs. The segmented, movable abdomen has six segments, each of which contains a pair of swimming limbs. In males, the first and second pair of abdominal limbs are long, groove-shaped and used as a copulatory organ. The female's first pair of limbs is greatly shortened. The abdomen ends with a caudal fin formed by a sixth pair of wide lamellar limbs and a caudal blade.
The gills of 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 cephalothoracic carapace. The circulation of water in the gill cavity is ensured 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 oral limbs passes through the short pharynx and esophagus into the stomach, which consists of two sections - chewing and filtering. On the inner walls of the chewing section of the stomach there are chitinous teeth, with the help of which food is ground. The food gruel is filtered through the bristles of the straining 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 the anus at its end.
Circulatory system typical of all arthropods - unclosed with a compact heart in the form of a pentagonal sac on the dorsal side of the cephalothorax.
Metabolic products are removed through excretory organs - paired green glands that lie at the base of the head and open outward at the base of the antennae. In their structure, the glands resemble modified metanephridia, which remove metabolic products from the body cavity.
Cancer's eyes are complex. They consist of large number 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 - antennae, and the organs of smell are short whiskers - antennules. At the base of the short mustache is the organ of balance.
At the end of winter, females lay fertilized eggs on their abdominal limbs. At the beginning of summer, the eggs hatch into crayfish, which remain under the protection of the female for a long time, hiding on her abdomen on the underside. Young crayfish grow rapidly and molt several times a year; adults molt only once a year. The cancer then produces soft chitin. After some time, it becomes saturated with lime, hardens, and the growth of the cancer stops until the next molt.
The role of crustaceans in nature and their practical significance. Crustaceans have great importance in nature and human economy. Countless crustaceans inhabiting sea and fresh waters serve as food for many species of fish, cetaceans and other animals. Daphnia, cyclops, diaptomus, bokoshavy - excellent food for freshwater fish and their game. Many small crustaceans feed by filtration, that is, they strain out the food suspension with their thoracic limbs. Thanks to their nutritional activity, it lightens natural water and its quality improves.
Many large crustaceans are commercial species, such as lobsters, crabs, lobsters, shrimp, and crayfish. Medium-sized marine crustaceans are used by humans to prepare nutritious protein paste.
Crustaceans, or crayfish, evolved from trilobite-like arthropods that moved to faster movement at the bottom of reservoirs and in the water column. Due to more in an active way life, the organization of crustaceans has become significantly more complex compared to their ancestors. This is a large and diverse class, representatives of which live in marine, fresh and brackish waters. Only a few crustaceans live on land, but only in damp places.
External building. The structure of crayfish (see Fig. 75, 80) is very diverse. Division of the body into sections different groups not similar. Often the head and thoracic regions merge together to form the cephalothorax, to which the articulated abdomen is connected. The size of the body varies widely: many forms - 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 - endocuticle, and the outer - exocuticle (Fig. 78). The latter is impregnated with tannins and is therefore very durable. During molting, the endocuticle dissolves and is absorbed by the hypodermis, but the exocuticle is insoluble and is completely shed. Large crayfish are covered with strong shells. Small forms may also have armored 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), the structure and functions of which 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, forming a ladder rather than a chain (see Fig. 72); in other crustaceans, the brain becomes more complex (in varying degrees in different groups), the abdominal cords form a chain, the nodes of which, as the concentration of the body increases, can be connected until all the nodes merge into one (see Fig. 72). The behavior of the highest representatives of the class, who are, as a rule, active predators, reaching a very large value, is greatly complicated and is ensured by progressive changes in the entire nervous system. The organs of touch in the form of sensitive bristles are scattered throughout the body, but there are especially many of them on the antennae. The organs that perceive chemical irritations are quite well developed; in large crayfish they are concentrated mainly on the antennae of the first pair. Equilibrium 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. Complex, or faceted, eyes (Fig. 79) consist of large quantity individual eyes, or ommatidia. Each ommatidium consists of a cornea (the transparent part of the chitinous cuticle), a crystal cone - an elongated transparent body, to which are adjacent 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 pigment cells that isolate the ommatidia from each other, and until nerve cells not coming. The latter perceive only those rays that fall perpendicular to the surface of the ommatidium. Thus, each ommatidia perceives only part of the object, yet the ommatidia perceive the entire object. The image of an object in the compound eye is composed of individual parts it is reminiscent of mosaic paintings (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 due to the strong tuck of the abdomen under the chest. In crayfish, unlike terrestrial arthropods, two-branched limbs are widespread, which, together with the setae, have a wide surface and are convenient for use as oars. In large crayfish, for example the crayfish, 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, are good for 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 sac. In a number of small forms, the heart is absent and the movement of blood is caused by intestinal movements, as well as movements of the whole body. The development of the network of blood vessels mainly depends on the size of the body: in large crayfish it can be quite well developed, in small crayfish it can be completely reduced.
Respiratory system. The respiratory organs of most crustaceans are gills, which are appendages of legs 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 oxygen absorption occurs through the surface of the body, mainly in its thinner places.
Excretory system. The excretory system is represented mainly by a pair, rarely more, metanephridia. Reduction in the number of these organs compared to annelids, in which they are numerous, is explained mainly by the fact that in crustaceans the body cavity is continuous, not divided by partitions, like in ringlets, and it is enough for them to have a small number excretory organs, but more complexly arranged, divided into a number of sections (Fig. 81). In higher crayfish, metanephridia reach especially great complexity; they are large (about 1 cm or more) and open at the base of the antennae of the second pair and are therefore called antennal. In other crayfish, the metanephridia are simpler in structure, they are smaller (see Fig. 80, A) and open at the base of the second pair of mandibles, or maxillae, 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 energetic work in some - antennae, in others - oral limbs, in others - thoracic legs, creating continuous flow of water. In the crustacean Daphnia, the hind thoracic legs beat 200-300 times per minute and ensure food enters the mouth. Large crayfish (see Fig. 73) capture prey using legs armed with claws.
Crustaceans, like all arthropods, have limbs that surround the mouth and perform a number of functions. The oral extremities of crayfish 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 for grinding food, and two pairs of lower jaws, which also serve for mechanical processing of food. In addition, three pairs of jaws, already located on the chest, help to hold food and convey 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 are used for 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 liver glands of vertebrates, since it secretes juice that facilitates 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, it is more correct to call the digestive gland of crayfish pancreatic-hepatic. In small crustaceans these glands are moderately developed, in the form of hepatic processes (see Fig. 80, A, 10); in large crustaceans 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, structure of limbs, etc. Parthenogenesis is widespread in some groups of lower crayfish. In cladocerans, which include many species (for example, various daphnia) that serve as food for fish, most During the warm season, only females are found that lay unfertilized eggs, from which new crustaceans quickly develop. Males usually appear before the onset of the cold season or other unfavorable conditions. Females fertilized by males lay eggs surrounded by strong, thick shells that do not develop until the following year. Many crayfish carry eggs on their abdomen or in a special brood chamber (see Fig. 80, A).
Development. Development with transformation or direct. In lower crustaceans that develop with metamorphosis, larvae called nauplii(Fig. 82). These larvae have three pairs of legs and one eye. In higher crayfish living in the sea, the eggs mostly hatch into larvae called zoea (Fig. 82). Zoeys have larger number limbs than nauplii, and two compound eyes; they are lined with spines, which increase their surface and make it easier to float in the water. Other types of larvae are also known that occupy an intermediate position between nauplius and zoea or between zoea and adult form. In many lower freshwater crustaceans and crayfish, development is direct.
The growth of crayfish is always associated with molting; For example, crayfish during the first year of its life it molts 10 times and therefore grows quickly (from 0.9 to 4.5 cm), during the second year it molts 5 times, during the third - only twice, and then females molt once a year , and males - 2 times. After 5 years they hardly grow; live 15 - 20 years.
Origin. Crustaceans originated, as noted above, from arthropods close to trilobites. In connection with adaptation to a more active and complex lifestyle, their body differentiation into sections increased, many segments merged, i.e. the concentration of the organism increased; has become more complicated nervous system; the structure of the limbs (generally the same among trilobites) became diverse in connection with the performance of different functions; the intensity of work of other organ systems has increased.
Lower Crustaceans
Subclass Gill-footed
The most primitive. These small crustaceans have leaf-shaped legs and are used equally for movement and breathing. They also create a current of water that carries food particles to the mouth. Their eggs easily tolerate drying out and wait in the soil for the new rainy season. Artemia is an interesting branchiopod: 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 (jaws)
Representatives of the barnacle order are amazing: sea acorns and barnacles. These sea crayfish switched to a sedentary lifestyle in houses made of limestone plates. The larva is a typical nauplius, sinks to the bottom and attaches itself with antennules. The antennules and the entire anterior part of the head turns into an organ of attachment (a long fleshy stalk in sea ducks, or a flat wide sole in sea acorns), the antennae and compound eyes atrophy, the thoracic legs extend into long two-branched “antennae”, driving food to the mouth.