Higher and lower crustaceans. Class Crustacea
lower crustaceans
Subclass Gillpods
The most primitive These small crustaceans the legs are leaf-shaped 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. Of the branchiopods, artemia is interesting: it can live in salt lakes with a salt concentration of up to 300 g / l, and in fresh water dies in 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.
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 the head (these include: two pairs of antennae (antennae), a pair of lower jaws (maxillas) 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 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. Life cycle Crustaceans begin with a fertilized egg that 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 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).
blue cuban crayfish
Crustaceans live in aquatic or humid environments and are close relatives of insects, spiders, and other arthropods (type Arthropoda). The peculiarity of their evolutionary series is to reduce the number of metameric (identical) segments through their merging with each other and the formation of more complex body fragments. According to this feature and other characteristics, two groups are distinguished: lower and higher crustaceans. So, let's get to know these animals closer.
Lower and higher crustaceans: characteristic differences
The lower crustaceans differ in small, up to microscopic sizes. In addition, they do not have abdominal limbs, but only chest ones. Unlike primitive forms, higher crustaceans are characterized by a constant (6 pieces) number of identical body segments. For simply arranged crustaceans, the number of such formations ranges from 10 to 46. Moreover, their limbs, as a rule, are biramous. While, in some highly developed animals, this feature disappears. So, in crayfish, the thoracic limbs have one branch.
cherry shrimp
Shrimp Lysmata amboinensis and giant moray
The lower crustaceans are characterized by a softer chitinous cover. Some of them (daphnia, in particular) have transparent shells through which you can see internal structure. Respiratory system in higher crustaceans it is represented by gills. More primitive forms breathe the entire surface of their body, while the bloodstream in some may be completely lost. The nervous system of highly developed species with a variety of behavioral reactions has a complicated structure.
Daphnia (lat. Daphnia) - a genus of planktonic crustaceans
These animals are characterized by well-developed external formations that perform the function of balance (statocysts); bristles covering the entire body, increasing sensitivity; organs that capture the chemical components of the environment. Some lower crustaceans do not have a peripharyngeal ring, their brain is more primitive, while in more developed organisms the ganglia merge, their structure becomes more complex.
Lobster, he is a lobster (lat. Nephropidae)
Diversity of biological forms of lower and higher crustaceans
Shrimp "Red Crystal"
A special commercial role for humans is played by higher species of crustaceans, in particular crayfish, crab, lobster, spiny lobster, shrimp. useful product consisting of planktonic crustaceans Bentheuphausia amblyops, is krill meat. Has the same lifestyle Macrohectopus branickii living in Lake Baikal. Land lice living in wet soil, also belong to highly developed representatives.
Cambarellus patzcuarensis is an endemic type of crayfish
Amphipod Parvexa, an endemic crustacean that lives in about. Baikal
Cancer - mantis (lat. Odontodactylus scyllarus), also known as shrimp - mantis
And in more detail with various types belonging to this class, with lower and higher crustaceans, you will be introduced to new articles online magazine « Undersea world and all its mysteries":
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.
Crustaceans, or crayfish, evolved from trilobite arthropods that moved to faster movement at the bottom of reservoirs and in the water column. Due to more actively In life, 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 different groups 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. Small forms can 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. Near a number lower forms the central section of this system consists of a relatively simple brain and abdominal cords that form a ladder, not a chain (see Fig. 72), in the remaining crustaceans the brain becomes more complex (to varying degrees in different groups), the abdominal cords form a chain, nodes which, as concentration increases, bodies can connect 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 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 ommatidium are absorbed by the pigment cells that isolate the ommatidia from each other, and up to nerve cells not coming. 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 of most crustaceans are gills, which are leg appendages that have different shape: in small crayfish, these are rounded leaves (Fig. 80, A), in large crayfish (as, for example, in 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 compared to 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 annulus, and it is enough for them to have a small number of excretory organs, but more complex, divided 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 antennae 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 the main organic compounds- proteins, carbohydrates and fats: the liver of vertebrates plays big 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 next 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). Zoey 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 adult form. Many lower freshwater crustaceans and crayfish have direct development.
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; complicated nervous system; the structure of the limbs (generally the same in trilobites) became diverse due to the performance of different functions; the intensity of the work of other organ systems has increased.
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