A short report on cephalopods. Biology at the Lyceum. Genitals and skeleton
As a rule, the body of mollusks is characterized by bilateral symmetry. In this regard, the head and torso are distinguished. The shell is present only in ancient forms, and in other representatives it is rudimentary. From above, the entire body of the mollusk is covered with a mantle, which consists of one layer of epithelium and connective tissue. Some species may have chromatophores, thanks to which the body can change its color.
Digestive system
The digestive system of cephalopods has a rather complex structure. But special attention deserves a gulp, which is equipped with horny jaws in the form of a beak. In addition, salivary secretions that are secreted into the pharyngeal cavity are quite poisonous. Thanks to them, the mollusk immobilizes its prey.The stomach is a pouch that smoothly passes into the hind intestine. There is also an ink sac in which ink is produced, thanks to which the mollusk is able to hide when danger arises. The digestive system ends at the anus.
Excretory and circulatory systems
The excretory system includes two or four kidneys. As a rule, their number depends on the representative. The circulatory system is represented by a heart with one ventricle and two atria. The bibranchial representatives have two atria, and the fourbranchial representatives have four.Sense organs
The sense organs are represented by osphadia and eye sacs. In the bibranchial representatives, instead of osphadia, there are olfactory pits, and as an organ of vision - compound eyes, which resemble the structure of the eyes of mammals.Genitals and skeleton
All cephalopods are dioecious organisms. Their fertilization is spermatophore, that is, spermatophores act as germ cells. The development of the offspring occurs in the egg, which until hatching is located in the mantle cavity of the female. The skeleton of most cephalopods is represented by a cartilaginous skull.Nervous system
A special feature of mollusks is their complex nervous system. It is represented by the brain. In this regard, animals can be trained, have a very good memory and even distinguish geometric shapes.Cephalopods are perhaps the most amazing representatives of mollusks. They have a developed nervous system, three hearts, the ability to change color and penetrate almost any hole, and in terms of intelligence they could easily compete with some mammals.
Who are cephalopods?
Cephalopods are a class of mollusks, main feature which is the presence of many tentacles located around the head. These include about 800 species, which are divided into two main subclasses: Dibranchia and Nautiloids (four-branched). The first includes octopuses, cuttlefish, squids and others, the second includes only nautiluses and Allonautilus.
Their typical habitat is salt water. Cephalopods do not live on the surface of the earth, but some species can crawl outside for a short time to move from one body of water to another. They live in all oceans and are found both closer to the surface and at great depths. Most representatives of cephalopods prefer to stay close to the bottom, moving along it with the help of tentacle arms.
Appearance
The body of cephalopods is located above the head. In squids it has a cylindrical or cone-shaped shape, in cuttlefish it is flattened. The body of an octopus looks like a soft dome or bag. If necessary, it can be changed so that its owner can get into a narrow hole.
From above, the body of cephalopods is covered with a skin-muscular sac - the mantle, inside which all the internal organs are located. Outside, fins of various types extend from it. The head contains well-developed eyes and tentacles, which are also called arms. Representatives of bibranchs have 8 or 10 of them, while nautiloids have up to 100. They are located around the mouth and are equipped with suction cups that help them move over various surfaces, capture and hold prey.
Cephalopods also use tentacles to move along the bottom of a reservoir, but they also have another, “reactive” method for moving through the water column. On their head there is a hole in the siphon tube, the second end of which opens directly into the mantle cavity. During movement, the mollusk collects water in the mantle and, by contracting its muscles, splashes it out through the siphon. This creates a strong push, and the animal suddenly moves forward.
Skeleton
Cephalopods do not have a backbone or bones, but they do have an internal skeleton. It looks like a cartilaginous capsule containing the brain, as well as small processes at the base of the fins and tentacles. In quadribranchs, it consists of only one cartilage that supports the nerve centers.
Several million years ago, during the Cambrian period, cephalopods had an external shell. Shellfish occupied it only partially. The rest of the space was filled with gas or water in order to rise up in the water column or, conversely, sink down. Today, only nautiloids, the most ancient and primitive representatives of the class, have a shell. It contains up to 40 chambers and reaches a diameter of 15 to 25 centimeters. Female argonaut octopuses also have a single-chamber shell, which is distinguished by special lobes on their arms.
Internal structure
Cephalopods are considered the most developed creatures among all invertebrates. They do not have ears, but they have excellent vision (except for nautiluses), a sense of balance and a sense of smell. Only they have a closed circulatory system. It consists of two hearts that supply exclusively the gills, and one heart that works on the internal organs. The blood of mollusks is colorless, but when interacting with oxygen, it acquires a blue tint. This is due to the fact that instead of hemoglobin it contains the protein hemocyanin.
The nervous system of cephalopods is also highly developed, especially in bibranchs. It consists of nerve ganglia, the accumulation of which next to each other forms the brain. Octopuses are considered the most “intelligent” among mollusks. They remember people and objects well, are trainable, and can perform quite complex tasks. Without prompting, they understand that closed jar you can get the loot by opening it. They can establish temporary cooperation with other species and develop entire strategies to hunt more efficiently.
Octopus limbs often perform various manipulations on their own, without the control of the brain. He sends them only simple, poorly defined tasks, and they decide for themselves exactly how to move. They contain a huge number of nerve cells, due to which they are capable of semi-independent actions.
Experimenting with color and lighting
Cephalopods have many talents. One of them is bioluminescence. Many of them are able to emit a glow in the lower part of the body. This way they “erase” their own shadow and become invisible to prey or natural enemies.
Another ability is to change color, like chameleons. Their skin contains special cells of red, brown and yellow colors that stretch or contract when necessary. Lamellar cells - irridioscites, turn, changing their position towards the light to obtain blue and green shades. By combining the actions of two groups of cells in different ways, cephalopods are able to acquire almost any color or pattern.
Ink
Most cephalopods have an ink sac hidden in their mantle, which is necessary for protection from enemies. It is located near the animals' intestines and opens directly into the posterior opening. Inside the bag there is mucus, water and the ink itself. When in danger, the mollusk throws its contents towards the predator and disappears behind a spreading curtain. Ink colors range from black, blue-black and brown. Since ancient times, they have been used for writing and drawing, and the name “sepia” tone even comes from the name of the cuttlefish, whose ink is colored brown.
With about 130,000 species, molluscs are second only to arthropods in number of species and represent the second largest phylum of the animal kingdom. Mollusks are predominantly aquatic inhabitants; only a small number of species live on land.
Mollusks have a variety of practical meanings. Among them there are useful ones, such as pearl mussel and mother-of-pearl, which are mined to obtain natural pearls and mother-of-pearl. Oysters and some other species are harvested and even farmed for food. Some species are pests of agricultural crops. From a medical point of view, mollusks are of interest as intermediate hosts of helminths.
General characteristics of the type
Animals belonging to the mollusc type are characterized by:
- three-layer, - i.e. formation of organs from ecto-, ento- and mesoderm
- bilateral symmetry, often distorted due to displacement of organs
- unsegmented body, usually covered by a shell, whole, bivalve or consisting of several plates
- skin fold - a mantle that fits the entire body
- muscular growth - a leg that serves for movement
- poorly defined coelomic cavity
- presence of basic systems: movement apparatus, digestive, respiratory, excretory, circulatory systems, nervous and reproductive systems
The body of mollusks has bilateral symmetry; in gastropods (which includes, for example, the pond snail) it is asymmetrical. Only the most primitive mollusks retain signs of body segmentation and internal organs, in most species it is not divided into segments. The body cavity is secondary, presented in the form of a pericardial sac and a cavity of the gonads. The space between the organs is filled with connective tissue (parenchyma).
The body of mollusks consists of three sections - head, trunk and legs. In bivalves, the head is reduced. The leg, a muscular outgrowth of the abdominal wall of the body, is used for movement.
At the base of the body, a large fold of skin is developed - the mantle. Between the mantle and the body there is a mantle cavity in which there are gills, sensory organs, and the openings of the hindgut, excretory and reproductive systems open here. The mantle secretes a shell that protects the body from the outside. The shell can be solid, bivalve, or consist of several plates. The shell contains calcium carbonate (CaCO 3) and the organic substance conchiolin. In many mollusks the shell is more or less reduced (for example, in some cephalopods, in naked slugs, etc.).
The circulatory system is not closed. The respiratory organs are represented by gills or lungs formed by part of the mantle (for example, in pond snails, grape and garden snails, naked slugs). The excretory organs - the kidneys - are connected at their internal ends to the pericardial sac.
The nervous system consists of several pairs of nerve ganglia, interconnected by longitudinal trunks.
The phylum of mollusks includes 7 classes. The most important of them:
- gastropods (Gastropoda) - slow crawling snails
- bivalves (Bivalvia) - relatively sedentary mollusks
- cephalopods (Cephalopoda) - mobile mollusks
| Table 1. Characteristic features of bivalves and gastropods | ||
| Sign | Class | |
| Bivalve | Gastropods | |
| Symmetry type | Bilateral | Asymmetrical with reduction of some right organs |
| Head | Reduced together with related organs | Developed |
| Respiratory system | Gills | Gills or lung |
| Sink | Bivalve | Spiral twisted or cap-shaped |
| Reproductive system | Dioecious | Hermaphrodite or dioecious |
| Nutrition | Passive | Active |
| Habitat | Marine or freshwater | Marine, freshwater or terrestrial |
Class Gastropoda
This class includes mollusks that have a shell (snails). Its height ranges from 0.5 mm to 70 cm. Most often, the shell of gastropods has the form of a cap or spiral; only representatives of one family develop a shell of 2 valves connected by an elastic ligament. The structure and shape of the shell are of great importance in the taxonomy of mollusks [show] .
- Placospiral shell - a strongly twisted shell, the whorls of which are located in the same plane
- Turbo-spiral shell – the revolutions of the shell lie in different planes
- Right-handed shell - the spiral of the shell twists clockwise
- Left-handed shell - the spiral twists counterclockwise
- Cryptospiral (involute) shell - the last whorl of the shell is very wide and completely covers all previous ones
- Open spiral (evolute) shell – all whorls of the shell are visible
Sometimes the shell is equipped with a lid located on the dorsal side at the back of the leg (for example, in lawns). When you pull your leg into the sink, the lid tightly covers the mouth.
In some species that have switched to a swimming lifestyle (for example, pteropods and keelenopods), there is no shell. Shell reduction is also characteristic of some land gastropods living in the soil and forest litter (for example, slugs).
The body of gastropods consists of a well-separated head, legs and torso - an internal sac; the latter is placed inside the sink. On the head there is a mouth, two tentacles and at their base two eyes.
Digestive system. At the front end of the head is the mouth. It has a powerful tongue covered with a hard chitinous grater, or radula. With its help, mollusks scrape algae from the ground or aquatic plants. In predatory species, a long proboscis develops in the front part of the body, capable of turning out through an opening on the lower surface of the head. In some gastropods (e.g., cones), individual teeth of the radula can protrude from the mouth opening and are shaped like stylets or hollow harpoons. With their help, the mollusk injects poison into the victim’s body. Some predatory gastropod species feed on bivalves. They drill into their shells, secreting saliva containing sulfuric acid.
Through the esophagus, food enters the pouch-shaped stomach, into which the liver ducts flow. Then the food enters the intestine, which bends in a loop and ends on the right side of the body with the anus - the anus.
The nerve ganglia are collected in the peripharyngeal nerve ring, from which nerves extend to all organs. The tentacles contain tactile receptors and chemical sense organs (taste and smell). There are organs of balance and eyes.
In most gastropods, the body protrudes above the leg in the form of a large spirally twisted sac. On the outside it is covered with a mantle and fits closely to the inner surface of the shell.
The respiratory organs of mollusks are represented by gills located in the front part of the body and directed with their apex forward (prosobranchial mollusks) or located in the right rear part of the body and directed with their apex backward (opisthobranchial). In some gastropods (for example, nudibranchs), true gills were reduced. They develop so-called respiratory organs. cutaneous adaptive gills. In addition, in terrestrial and secondary aquatic gastropods, part of the mantle forms a kind of lung, numerous blood vessels develop in its walls, and gas exchange occurs here. The pond snail, for example, breathes atmospheric oxygen, so it often rises to the surface of the water and opens a round breathing hole on the right at the base of the shell. Next to the lung is the heart, which consists of an atrium and a ventricle. The circulatory system is not closed, the blood is colorless. The excretory organs are represented by one kidney.
Among gastropods there are both dioecious species and hermaphrodites, the gonads of which produce both spermine and eggs. Fertilization is always cross-fertilization, development, as a rule, with metamorphosis. Direct development is observed in all land, freshwater and some marine gastropods. Eggs are laid in long mucous threads attached to movable objects.
Belongs to the class of gastropods
- The common pond snail is often found on aquatic plants in ponds, lakes and rivers. Its shell is solid, 4-7 cm long, spirally twisted, with 4-5 curls, a sharp apex and a large opening - the mouth. A leg and head can protrude through the mouth.
Gastropods also include intermediate hosts of trematodes.
- The intermediate host of the cat fluke, Bithynia leachi, is widespread in freshwater bodies of our country. It lives in the coastal zone of rivers, lakes and ponds overgrown with vegetation. The shell is dark brown and has 5 convex whorls. Shell height 6-12 mm.
- The intermediate host of the liver fluke, the small pond snail (Limnea truncatula), is widespread in Russia. The shell is small, no more than 10 mm in height, forms 6-7 whorls. It lives in ponds, swamps, ditches and puddles, where it is often found in huge quantities. In some areas there are more than 1 million pond snails per hectare of swamps. When swamps dry out, pond snails burrow into the ground, surviving the dry time in the ground.
- The intermediate hosts of the lancet fluke are the terrestrial mollusks Helicella and Zebrina. Distributed in Ukraine, Moldova, Crimea and the Caucasus. Adapted to life in arid conditions; live in the open steppe on the stems of herbaceous plants. During hot weather, helicella often accumulate on plants in clusters, thus saving themselves from drying out. Helicella has a low-conical shell with 4-6 whorls; the shell is light, with dark spiral stripes and a wide rounded mouth. Zebrina has a highly conical shell with 8-11 whorls; the shell is light, with brown stripes running from the top to the base; the mouth is irregularly oval.
Class bivalve (Bivalvia)
This class includes mollusks with a shell consisting of two symmetrical halves, or valves. These are sedentary, sometimes completely motionless animals that live on the bottom of seas and freshwater bodies. They often burrow into the ground. The head is reduced. In freshwater reservoirs, toothless or pearl barley are widespread. Of the marine forms, oysters are the most important. Very large species are found in tropical seas. The shell of a giant tridacna weighs up to 250 kg.
Perlovitsa, or toothless lives on the silted and sandy bottoms of rivers, lakes and ponds. This inactive animal feeds passively. The toothless food consists of detritus particles suspended in water (the smallest remains of plants and animals), bacteria, unicellular algae, flagellates, and ciliates. The mollusk filters them from the water passing through the mantle cavity.
The body of the toothless fish, up to 20 cm long, is covered on the outside with a bivalve shell. There is an expanded and rounded front end of the shell, and a narrowed, pointed rear end. On the dorsal side, the valves are connected by a strong elastic ligament, which maintains them in a semi-open state. The shell closes under the action of two closing muscles - anterior and posterior - each of which is attached to both valves.
There are three layers in the shell - horny, or conchiolin, which gives it a brownish-green color on the outside, a middle thick porcelain-like layer (consists of prisms of carbonated lime; located perpendicular to the surface - the shell) and an inner mother-of-pearl layer (between the thinnest calcareous leaves there are thin layers of conchiolin). The nacreous layer is underlain on each of the two valves by a yellowish-pink fold of the mantle. The epithelium of the mantle secretes the shell, and in some species of freshwater and sea pearl oysters it also forms pearls.
The body is located in the dorsal part of the shell, and a muscular outgrowth extends from it - the leg. In the mantle cavity on both sides of the body there are a pair of lamellar gills.
In the rear part, both shell valves and mantle folds do not fit tightly to one another; two openings remain between them - siphons. The lower inlet siphon serves to introduce water into the mantle cavity. A continuous directed flow of water is carried out due to the movement of numerous cilia that cover the surface of the body, mantle, gills and other organs of the mantle cavity. Water washes the gills and provides gas exchange; it also contains food particles. Through the upper, outlet siphon, used water along with excrement is discharged outside.
The mouth is located at the front end of the body above the base of the leg. On the sides of the mouth there are two pairs of triangular oral lobes. The cilia covering them move food particles toward the mouth. Due to the reduction of the head in pearl barley and other bivalves, the pharynx and associated organs (salivary glands, jaws, etc.) are reduced.
The digestive system of pearl barley consists of a short esophagus, a pouch-shaped stomach, a liver, a long loop-shaped midgut and a short hindgut. A sac-like outgrowth opens into the stomach, inside which there is a transparent crystalline stalk. With its help, food is crushed, and the stalk itself gradually dissolves and releases the amylase, lipase and other enzymes contained in it, which ensure the primary processing of food.
The circulatory system is not closed; Colorless blood flows not only through the vessels, but also in the spaces between the organs. Gas exchange occurs in the gill filaments, from there the blood is sent to the efferent gill vessel and then to the corresponding (right or left) atrium, and from it to the azygos ventricle, from which two arterial vessels begin - the anterior and posterior aorta. Thus, in bivalves, the heart consists of two atria and one ventricle. The heart is located in the pericardial sac on the dorsal side of the body.
The excretory organs, or kidneys, look like dark green tubular sacs; they start from the pericardial cavity and open into the mantle cavity.
The nervous system consists of three pairs of nerve ganglia connected by nerve fibers. The sense organs are poorly developed due to the reduction of the head and a sedentary lifestyle.
Class Cephalopoda
unites the most highly organized mollusks leading an active lifestyle. Cephalopods include the largest representatives of invertebrates - octopuses, squids, cuttlefish.
The body shape of cephalopods is very diverse and depends on their lifestyle. Inhabitants of the water column, which include most squids, have an elongated, torpedo-shaped body. Benthic species, among which octopuses predominate, are characterized by a sac-like body. In cuttlefish living in the bottom layer of water, the body is flattened in the dorsoventral direction. Narrow, spherical or jellyfish-like planktonic species of cephalopods are distinguished by their small size and gelatinous body.
Most modern cephalopods do not have an external shell. It turns into an element of the internal skeleton. Only nautiluses retain an outer, spirally twisted shell, divided into internal chambers. In cuttlefish, the shell, as a rule, has the appearance of a large porous calcareous plate. Spirula retains a spirally twisted shell hidden under the skin. In squids, only a thin horny plate is retained from the shell, stretching along the dorsal side of the body. In octopuses, the shell is almost completely reduced and only small crystals of lime carbonate remain. Female argonauts (one of the species of octopuses) develop a special brood chamber, shaped very much like an external shell. However, this is only an apparent similarity, since it is secreted by the epithelium of the tentacles and is intended only to protect developing eggs.
One of the distinguishing features of cephalopods is the presence of an internal cartilaginous skeleton. Cartilage, similar in structure to the cartilage of vertebrates, surrounds the head cluster of ganglia, forming a cartilaginous capsule. Branches extend from it, reinforcing the eye openings and balance organs. In addition, supporting cartilage develops in the cufflinks, base of the tentacles and fins.
The body of cephalopods consists of a head with compound eyes, a crown of tentacles or arms, a funnel and a torso. Large, complex eyes are located on the sides of the head and are not inferior in complexity to the eyes of vertebrates. The eyes have a lens, cornea and iris. Cephalopods have developed not only the ability to see in stronger or weaker light, but also accommodation. True, it is achieved not by changing the curvature of the lens, as in a person, but by bringing it closer or further away from the retina.
On the head around the mouth opening there is a crown of very mobile tentacles, which are one part of a modified leg (hence the name). The vast majority of species have powerful suckers on their inner surface. Squids use tentacles to catch prey; male octopuses use one of the tentacles to carry reproductive products. During the breeding season, this tentacle is modified, and during the mating period it breaks off and, due to its ability to move, penetrates into the mantle cavity of the female.
The other part of the leg turns into a funnel, which plays an important role in movement. It grows to the ventral side of the body, opening at one end into the mantle cavity, and at the other into the external environment. The mantle cavity in cephalopods is located on the ventral side of the body. At the junction of the body and the head, it communicates with external environment transverse abdominal foramen. To close it, in most cephalopods, paired semilunar fossae are formed on the ventral side of the body. Opposite them, on the inner side of the mantle, lie two hard tubercles reinforced with cartilage, the so-called. cufflinks As a result of muscle contraction, the cufflinks fit into the semilunar recesses, tightly fastening the robe to the body. When the abdominal opening is open, water freely penetrates into the mantle cavity, washing the gills lying in it. After this, the mantle cavity closes and its muscles contract. The water is forcefully pushed out of the funnel lying between the two cufflinks, and the mollusk, receiving a reverse push, moves forward with the rear end of the body. This method of movement is called reactive.
All cephalopods are predators and feed on a variety of crustaceans and fish. They use tentacles to capture prey, and powerful horny jaws to kill. They are located in the muscular pharynx and resemble the beak of a parrot. The radula is also placed here - a chitinous ribbon with 7-11 rows of denticles. 1 or 2 pairs of salivary glands open into the pharynx. Their secretion contains hydrolytic enzymes that break down polysaccharides and proteins. Often the secretions of the second pair of salivary glands are poisonous. The venom also helps immobilize and kill large prey.
The intestine is branched, with digestive glands. In many species, just before the anus, the duct of the ink gland opens into the lumen of the hindgut. It secretes a dark secretion (ink) that can cloud a large amount of water. The ink serves as a smoke screen, disorienting the enemy, and sometimes paralyzing his sense of smell. Cephalopods use it to escape from predators.
The circulatory system is almost closed. A heart with 2 or 4 atria, also 2 or 4 kidneys, their number is a multiple of the number of gills.
The nervous system has the highest organization with developed structures of touch, smell, vision and hearing. The ganglia of the nervous system form a common nervous mass - a multifunctional brain, which is located in a protective cartilaginous capsule. Two large nerves arise from the posterior part of the brain. Cephalopods have complex behavior, have good memory and exhibit the ability to learn. Because of the perfection of their brains, cephalopods are called “primates of the sea.”
The unique cutaneous photoreceptors of cephalopods respond to the slightest changes in light. Some cephalopods are capable of glowing due to the bioluminescence of photophores.
All cephalopods are dioecious animals; Some of them have well-defined sexual dimorphism. Males, as a rule, are smaller than females, armed with one or two modified arms - hectocotyls, with the help of which "packets" with seminal fluid - spermatophores - are transferred during the copulation period. Fertilization is external-internal and occurs not in the female’s reproductive tract, but in her mantle cavity. It involves the capture of sperm by the gelatinous membrane of the eggs. After fertilization, females attach clusters of eggs to bottom objects. Some species take care of their offspring and protect developing eggs. The female protecting the offspring can starve for more than 2 months. In octopuses, cuttlefish and nautiluses, a minicopy of the parents hatches from each egg, only in squids development occurs with metamorphosis. The young grow quickly and often reach sexual maturity by one year.
The meaning of shellfish
Freshwater pearl mussel shells with a mother-of-pearl layer thickness of about 2.5 mm are suitable for making mother-of-pearl buttons and other jewelry. Some bivalves (mussels, oysters, scallops), grape snail from gastropod mollusks (in some European countries it is bred in snail farms), are consumed as food; among cephalopods, squid is especially valuable in terms of calorie content and protein composition (more than 600 thousand of them are caught annually in the world . T).
River zebra mussels are found in huge quantities in the reservoirs of the Volga, Dnieper, Don, in lakes, estuaries of the Black Sea, and desalinated areas of the Azov, Caspian and Aral seas. It grows over stones, piles and various hydraulic structures: watercourses, technical and drinking water supply pipes, protective gratings, etc., and its quantity can reach 10 thousand copies per 1 m2 and cover the substrate in several layers. This makes it difficult for water to pass through, so constant cleaning of zebra mussel fouling is necessary; use mechanical, chemical, electrical and biological control methods. Some bivalves bore passages in the bottoms of ships and wooden parts of port facilities (shipworm).
Pearl barley and some other bivalves play an important role in marine and freshwater biocenoses as natural water purifiers - biofilters. One large pearl barley can filter 20-40 liters of water per day; mussels inhabiting 1 m2 of the seabed can filter about 280 m3 of water per day. In this case, mollusks extract organic and inorganic substances from contaminated water, some of which are used for their own nutrition, and some are concentrated in the form of lumps that are used to feed microorganisms.
Thus, mollusks are one of the most important parts of the self-purification system of a reservoir. Particularly important in the system of biological self-purification of water bodies are mollusks, which have special mechanisms of resistance to pollution of water bodies with toxic substances and mineral salts, and are also adapted to living in water with a reduced amount of oxygen. The basis of the molecular mechanism of such adaptation are carotenoids contained in the nerve cells of mollusks. Pearl barley and other filter-feeding mollusks need protection. They can be bred in special containers and used to clean artificial reservoirs from pollution, waste disposal and obtain additional food products.
Shellfish fishing is especially important in Japan, the USA, Korea, China, Indonesia, France, Italy, and England. In 1962, the production of mussels, oysters, scallops and other bivalves amounted to 1.7 million tons; by now, natural reserves of valuable edible shellfish have been depleted. In many countries, marine and freshwater mollusks are bred artificially. Since 1971, in the northwestern part of the Black Sea, mussels have been bred in an experimental farm (productivity is 1000 quintals of mussels per year), research on mussel breeding is also carried out in the basins of other seas washing the shores of our country. Shellfish meat is easily digestible, it contains a lot of vitamins, carotenoids, microelements (iodine, iron, zinc, copper, cobalt); it is used as food by the population, as well as for fattening domestic animals. Filter-feeding mollusks can also be used in a biomonitoring system to monitor the chemical composition of water in reservoirs.
Cephalopods, common in all seas except desalinated seas, despite the fact that they are predators, often themselves serve as food for many fish and marine mammals (seals, sperm whales, etc.). Some cephalopods are edible and are subject to commercial fishing. In China, Japan and Korea, the use of these animals as food goes back centuries; in Mediterranean countries it also has a very long history. According to Aristotle and Plutarch, octopuses and cuttlefish were common food in ancient Greece. In addition, they were used in medicine, perfumery and in the manufacture of first-class paints. Currently, innate programs of complex behavior are being studied in the laboratory on cephalopods.
INTRODUCTION
Class cephalopods (Cephalopoda, from Greek kephale - head, pus, podos - leg). These marine animals, which include squid, octopus, nautilus and cuttlefish, are considered the most developed of all mollusks.
In ancient times, cephalopods were much more numerous and diverse, the number of their species was close to 10,000.
All cephalopods are marine animals. They are found in the northern and Far Eastern seas.
The oldest of the cephalopods were nautiloids (Nautiloidea) and ammonites (Ammonoidea), named after the ancient Egyptian god Amun, whom the priests depicted with the head of a ram. A coiled ram's horn, similar to an ammonite shell, was the emblem of the ram god. Both nautiluses and ammonites lived in massive spiral or straight shells, divided into chambers and filled with gas. The shells were both crowbar and float. The animals, like inflatable boats, drifted freely on the waves, which contributed to their wider distribution.
This class of mollusks has gone farthest from its ancestors and has achieved a unique degree of development among invertebrates. These animals achieved fame as a result of stories of huge monsters capable of swallowing ships - an activity that is completely contrary to the nature of these creatures.
Most species of this class have partially or completely lost the calcareous shells characteristic of mollusks. Only the nautilus, a four-branched cephalopod with a multi-chambered shell, survived as a member of the once dominant group with an external shell.
Cephalopods have a number of unique features: great activity, method and speed of movement, an unusually highly developed nervous system, the rudiments of “intelligence,” a set of means of defense and attack.
CLASS Cephalopods
Molluscs or soft-bodied mollusks are a separate, large (unites over 130 thousand species and ranks 2nd after the type of arthropods) type of highly organized invertebrates, originating from annelids, appearing for the first time in the Cambrian period. Evidence of origin from ancient annelids is the similarity in the structure of the larvae of marine gastropods and the larvae of marine polychaetes, and also some primitive mollusks have common structural features with annelids. These are predominantly aquatic animals (they live in seas, oceans, and fresh water bodies) and only a few have adapted to life on land. The phylum includes 7 classes, of which the most common are Gastropods, Bivalves and Cephalopods.
Cephalopods are the most developed among mollusks. The name “cephalopod” means that the muscular organ of movement, the leg, is located in their head region. In these animals, the leg has been transformed into a whole crown of tentacles. In living cephalopods, the internal shell either disappeared completely or was reduced to a transparent arrow in squids.
There are 800 living and 8,000 extinct species of cephalopods. They got the name because there are limbs on the head and them - tentacles with suction cups surrounding the mouth opening, and legs forming a funnel.
Cephalopods include predatory mollusks that have tentacles, but do not have shells, but only their remains. However, there are exceptions. For example, ammonites. They have shells that are similar in appearance to those of gastropods. Ammonites went extinct many millions of years ago. The closest modern relative of the ammonite, the nautilus, lives in the Pacific Ocean. Like ammonites, the nautilus shell is divided into chambers. By adjusting the gas content in the chambers, the nautilus floats and submerges. He swims backwards with his head down.
On the large head of cephalopods there are eyes and a mouth with horny jaws and a radula; it is surrounded by either 8 or 10 arms or many tentacles. In addition to the usual tentacles, they also have two long hunters. Dimensions vary from a few centimeters from 1 cm to 18 m. All species are dioecious; fertilization is internal. The eggs, surrounded by gelatinous capsules, hatch into miniature, adult-like immature individuals.
Cephalopods are bilaterally symmetrical animals with an external or internal rudimentary shell. On the ventral side of the internal mass lies an ink sac with a duct that drains into the intestine. In the upper part of the mantle cavity there are gills - one on either side of the visceral mass. The blood is driven by three hearts: the main one, consisting of a ventricle and two atria, and two gills. The heart rate of an octopus reaches 50 beats per minute. The circulatory system is almost closed. Skin and muscles have capillaries. The blood of cephalopods is blue due to the presence of the respiratory pigment hemocyanin, which contains copper. The excretory organs are the kidney sacs, the appendages of the gill hearts, and the gills themselves. The nervous system is more complex than that of other invertebrates. The ganglia are very close together and form a large brain. Their sense organs are highly developed. The eyes of cephalopods resemble the eyes of fish in the complexity of their structure, and are not inferior to human eyes in visual acuity. The eyes are usually located in the recesses of the cartilaginous head capsule and have a cornea, an iris with a pupil capable of contracting and dilating, a lens and a retina. There is even an eyelid that can cover the eye (in the squid - onychotheuthids).
Cephalopods are the most unusual, largest and most highly organized of mollusks; they do not even have a shell, so typical of soft-bodied ones.
These animals live exclusively in oceans and seas, the salt content of which is at least 33%. Therefore, they cannot be found either in the Black or Baltic Seas.
All cephalopods are predators.
The meat of squid and octopus is a human food product. Their fishing is especially developed in Japan, China and Korea.
These animals are so highly organized that they are called primates of the sea.
Smaller species of cephalopods can be found in shallow waters above sandy bottoms or in caves among reefs. During the day they hide with the help of excellent camouflage covering their robes, move reluctantly even if disturbed by divers, and never run very far.
At night, they turn into fast and efficient hunters. Giant deep-sea squids, whose body length reaches six meters and the length of their limbs (tentacles) up to ten meters, rarely rise to the surface, but are relatively common at depths, where they lead a solitary lifestyle and are predators.
Cephalopods (cephalopods = head + legs) are classified into only two orders: one order includes all decapod species, such as cuttlefish and giant squid, and the second order is eight-legged species or octopuses. But, since the limbs are artfully folded when swimming, the octopus is lighter can only be determined because its tentacles are always longer than its body.
There are, however, deep-sea varieties that have from eight to ten tentacles, and these link the two orders.
Cephalopods are the only deep-sea invertebrates that have evolved good visual memory, and they use it better than any vertebrate. Their eyes, which have more than 70 million visual cells, surpass human eyes in visual acuity.
They allow the animal to distinguish colors, and can adapt to different distances by changing focus. Moreover, cephalopods can determine the condition of the seabed by feeling it with their tentacles - this information is very important when choosing camouflage.
Cephalopods are capable of learning. Laboratory tests have shown that they can use tools and learn from experience - abilities that have so far only been discovered by apes.
Their special form of defense - throwing out a stream of ink - is another proof of the complexity of the behavior. The dark cloud of ink serves to confuse the predator. When the latter swims into this cloud, he temporarily loses orientation. You can find one octopus, skillfully camouflaged to match the sand on which it lies, and a few meters away from it, a second one, taking on the colors of the rough, dark fragment of the reef on which it is located.
For such absolute adaptation of appearance to its environment, the octopus needs two sources of information: data about color, which is provided by its eyes, and data on texture or surface structure, which is provided to it by its senses of touch. When they hunt fish, they also respond to each action of the prey by changing color, becoming lighter or darker. The darker color serves as an indicator of aggression. The cuttlefish, if frightened, reacts in a very special way: it becomes pale, but there are two very dark spots on its back, apparently designed to convince the intended enemy that these are the eyes of a much larger animal buried in sand.
Only males wear a dark striped mating outfit during the courtship period, while females have less noticeable stripes. Using a specially adapted tentacle, the male transfers sperm into the cavity of the female's mantle. After spawning, octopuses and some other cephalopods guard the eggs, ventilate them to provide oxygen, and, if necessary, help the young hatch. Cephalopods are carnivores; they feed on crustaceans, fish or molluscs. Their horny parrot-like jaws and rough tongue are the tools with which they eat their prey. They often engage in fierce fights with crabs or other crustaceans, whose claws pose a real danger to them.
Cephalopods have many enemies. Moray eels, conger eels and stingrays lurk between the reefs. In the open sea, cephalopods become prey for sharks and catfish whales, and in shallow waters they are threatened by birds and seals.
This strict natural selection means that only animals with a high level of development of behavioral stereotypes survive.
The nervous system in cephalopods, compared to other mollusks, achieves the greatest development: the nerve ganglia merged and formed a large brain. Their sense organs are highly developed. The eyes of cephalopods resemble the eyes of fish in the complexity of their structure, and are not inferior to human eyes in visual acuity.
Most cephalopods alternate between walking and swimming. Walking is done, pull the organ on your hands; swimming—especially in squid—involves removing a jet of water from the mantle cavity.
Water is drawn into the mantle cavity by relaxation of the teres muscle, which results in expansion of the mantle. Water enters around the neck area or mantle aperture, and through a pipe of some variety. When the mantle is contracted, the aperture is closed by a gripping mechanism and contraction of the anterior circular muscle. The water is thus squeezed out through the pipe. This mobile body is similar in construction to a jet plane; it can be turned in any direction, giving greater flexibility of movement to the animal.
Squid, cuttlefish and octopus have the ability to change color quickly and dramatically, often harmonizing with their environment. The basic color control mechanism consists of tiny sacs of pigments, each a different color, embedded just below the surface of the animal's skin. These amazing ncuro-controlled pigment sacs are called chromatophorcs. The pigment in these sacs can be brown, white, yellow or even irri blue. A little later, the creature would slide into the shadows, its white pouches contracting, and its dark brown pouches expanding. Now it would suddenly seem as dark as his new surroundings. In the shadows or at night, it can take on colors ranging from dark brown to deep red. It appears that cephalopod color manipulation has both defensive and offensive (offensive) aspects. Indeed, the use of these skills may even vary between the octopus and its tree-swimming relatives, the cuttlefish and squid. The octopus, being a ground dweller and shellfish eater, probably does not use its camouflage offensively. Cuttlefish and squid, however, can hover, motionless in open waters, camouflage themselves with neutral colors, and trap missing fish who come too close. Octopus and cuttlefish have visible pumps, or jets, through which they can remove water. When a cephalopod decides to flee, it indicates pumping forward. This allows it to recoil from danger swiftly. Indeed, octopuses and other cephalopods even have special defense techniques to escape when surprised by an attacker in open waters. They remove the ink, creating an instant decoy and allowing them to escape. The cloud of ink seems to paralyze the predator's senses of sight and smell.
These animals have an extremely advanced excitatory system, in fact, they are often referred to as brainy. They also have well developed eyes. The eye size of cephalopods is record-breaking.
Cephalopods are carnivores and have a pair of powerful beak-like jaws that can crush their prey. Their arms are weapons with tentacles and powerful suction cups, used to catch prey and bring it to the mouth.
The body of any mollusk has digestive, circulatory, excretory and other organ systems. The digestive system begins with the oral cavity, which passes into the pharynx (with a grater), esophagus, stomach with the digestive gland, liver, middle and hind intestine, which opens outward through the anus into the mantle cavity. Many species of mollusks have salivary glands .
The circulatory system of mollusks is not closed. It most often consists of two-chamber heart and blood vessels branching from it. The nervous system is formed by several pairs of ganglia with nerves. Metabolic products that are unnecessary for the body come from the blood of mollusks to the kidneys, and then into the mantle cavity and are removed outside. There may be one, two or four kidneys.
Remarkable advanced courtship behavior in cephalopods, especially squids, involves complex visual displays of movement and changes in color pattern. Males indicate that they are ready to breed by adopting a distinctive striped pattern, and displaying their fourth arm in a Hat customized manner. Hello - the fourth arm is in the squid, and the octopus is structurally modified to remove balls of sperm from its own body and place them inside the cavity of the female's mantle. From there, the sperm subsequently makes its way through the tubes. Squids do not worry about their fertilized eggs, which are laid on vegetation. In octopuses, however, the eggs are guarded by the mother. When thoughtful of her young, the mother eats a little, and her feeding habits change. About 10 days after her eggs hatch, she dies.
Cephalopods are dioecious. They usually reproduce once in a lifetime, laying large eggs on underwater objects. Development is direct: a small mollusk emerges from the egg, similar to an adult.
Moving on to the penultimate point of the plan, the teacher asks: who bought and ate canned squid? Shows a tin can with a picture of squid. But it turns out that squid is not only canned, but also dried, fried, and boiled. Even in ancient Rome, skillfully prepared octopus was a common food. Recently, human “gastronomic” interest in cephalopods has increased sharply, since their meat is a complete protein food that can replace fish. Squids can be found in the sea in schools of thousands and are easy to catch with nets. Octopuses are caught individually - with spears or using “jug traps”. In some countries, paint and ink are made from the ink liquid of cephalopods.
In the process of evolution, cephalopods have acquired many interesting abilities that help them occupy first place among mollusks. The most interesting of them is the ability to change body color. The leaders of camouflage and camouflage are cuttlefish, which look like flattened squids. They can not only change their color to match the color of the soil and stones, but also become striped and spotted. Cephalopods owe this amazing ability to special cells in their skin that look like paint bubbles - chromatophores (from the Greek - color). Each chromatophore has the finest muscle fibers that can compress or stretch it. The diameter of this cell can change 60 times in a split second! As soon as the chromatophores with the black pigment melanin are flattened into a pancake, the octopus will immediately darken and become invisible against the background of the black rock. And if all its chromatophores shrink, the octopus will turn white.
The ability to turn colorless almost instantly is necessary for the mollusk to perform the disappearing trick. The fact is that all cephalopods have a so-called ink sac. This gland produces a significant amount of the same melanin. At the moment of danger, the octopus sharply contracts its ink sac and an ink cloud flies out of it, slightly reminiscent of the octopus itself in shape. Himself<бомбометатель>at this time it suddenly turns pale and jerks away to the side. The predator is fooled. Instead of an octopus, he only grabs a dark cloud. Droplets of discarded paint burst and the cloud blurs, forming a real<дымовую завесу>!
Cephalopods can not only change their color, but also... glow! This ability is especially developed in deep-sea species that live in the darkness of the underwater night. They do not glow themselves, but thanks to special bacteria capable of emitting a faint glow. These bacteria live in cephalopods in special<карманах>, where they get from sea water. Such bags of bacteria are called photophores (from Greek - light and Latin phero - to carry). Mollusks provide housing for bacteria, and with their light they help attract prey and send signals to fellow tribesmen. The glow of living organisms is called bioluminescence (from Greek - life and Latin lumen - light). This light is much more economical than light from electric bulbs. In luminescent bacteria, more than 90% of their energy is converted into light rays. A burning light bulb wastes the same amount of energy on useless heat.<Огоньки>squid and octopus burn for years without recharging and do not heat the water at all!
Finally, cephalopods have another amazing feature. In many of their species, males differ in appearance from females! This phenomenon, called sexual dimorphism, is very rare among simply organized groups invertebrates. And in many cephalopods, males differ from females in size, appearance, and behavior. For example, among Argonaut octopuses, males are much smaller than females. They fertilize eggs in an extremely original way. One of the male Argonaut's tentacles contains packets of sperm. During the mating season, it breaks away (remember autotomy!) and swims on its own in search of a female. Just miracles!
CONCLUSION
Giant octopuses, along with man-eating white sharks, serve as a symbol of horror and fear in underwater world. Many films and books convince us of the mortal threat posed by these funny and mysterious animals. Yes, large octopuses can really become dangerous if you disturb them in their native den or tease them to the point of “white heat.” Usually, even specimens of respectable size try to sneak away or hide from humans, camouflaging themselves and changing their color to match the color of the bottom. There is information about the existence of giant squids up to 30 m long or huge octopuses up to 10 m in tentacle coverage, but there is no real evidence of these monsters attacking humans - at least at the present time. Perhaps we don’t meet them because we live in different layers of the world’s oceans: they are in the depths of the sea, and you and I are splashing at the surface of the water.
The real mortal danger to humans is not giant octopuses, but tiny octopuses, most numerous in the Indian Ocean and off the coast of Australia. When such an octopus is angry, blue, purple and purple rings appear on its body. Some tourists, seeing these cute little animals for the first time, place them in their palms to admire the play of changing colors. Retribution comes immediately: an imperceptible prick of the octopus's poisonous beak causes muscle paralysis, and the victim can quickly die from suffocation. Octopuses live in shallow water, so any child snorkeling can spot them and be eager to catch them. Unfortunately, this is very easy to do, since the small mollusk is not able to escape as quickly as its larger counterparts.
The person injected must immediately undergo artificial ventilation and, possibly, chest compressions until a qualified doctor arrives.
Invertebrate animals, due to the fact that there are no bones in their bodies, move much more slowly than vertebrates.
Cephalopods are the most unusual, largest and most highly organized of mollusks. A number of unique features - great activity, method and speed of movement, an unusually highly developed nervous system, the beginnings of "intelligence", a set of means of defense and attack - puts cephalopods above all other groups of invertebrates and allows them to compete with vertebrates.
At first glance, cephalopods - octopuses, cuttlefish and squids - do not resemble other mollusks in any way. They (except Nautilus) there is not even a shell, so typical of soft-bodied animals.
Rice. 22. Squid Wonderful lamp Lycoteuthis diadema.
These mollusks received the name “cephalopod” because they have limbs on their heads - tentacles, with the help of which many animals can move along the bottom. In addition, it has been established that the limbs of cephalopods developed from the legs of an ancient ancestor.
The sizes of cephalopods vary greatly. Among them there are dwarfs, whose mantle length is less than 1 cm, and giants, which include squids. The length of their body including tentacles can reach 18 m.
Cephalopods are exclusively marine animals. They play a very important role in ocean life. Being predators, they eat a huge amount of crustaceans, fish and, in turn, themselves serve as food for many marine vertebrates - fish, birds, pinnipeds and whales. The most important enemy of cephalopods is the giant toothed whale - the sperm whale.
Cephalopods are bilaterally symmetrical animals.
In cuttlefish the body is flattened, in squid it is cylindrical, pointed towards the rear end, fusiform, in octopuses it is sac-shaped. In all cephalopods, the body is covered with a skin-muscular sac - a mantle, which encloses the internal organs. On the sides of the mantle of squid, cuttlefish and pelagic octopuses there are fins, which come in a wide variety of shapes and sizes, and serve the mollusks for swimming and as rudders. The head of most cephalopods is large, often somewhat wider than the mantle and separated from it by a cervical interception, but in octopuses the head is fused with the mantle. The head bears eyes, often very large, especially in deep-sea squids, and tentacles that surround the mollusk’s mouth like a crown.
The inner surface of the tentacles (arms) is lined with suction cups. They are located in 1–4 (rarely more) longitudinal rows.
A muscular conical tube is attached to the underside of the head and sometimes grows, with its base extending into the mantle cavity. This funnel, or siphon, the main propulsion device of the cephalopod, its “jet engine”.
The mouth opening of cephalopods is small. The pharynx is muscular, equipped with two strong chitinous jaws, reminiscent of a parrot's beak and called " beak" In the oral cavity on a special tongue-like protrusion - odontophore a radula is placed - a chitinous ribbon seated in rows of small denticles. With the help of the radula, food that enters the mollusk’s mouth and is moistened with saliva is transported further into the esophagus. A thin tube, the esophagus, stretches from the throat to the stomach, piercing the brain and liver on its way to the stomach. Therefore, cephalopods, despite their great appetite, cannot swallow their prey whole, but are forced to crush it into small pieces with their “beak” before putting it in their mouth. The eaten pieces of food then enter the muscular stomach, which receives digestive juices produced by the liver and pancreas. The enzyme activity of these glands is very high, and food is digested within 4 hours. Absorption occurs in the caecum of the stomach - tsekume, as well as in the liver. Undigested food remains enter the intestine and are thrown out.
In the upper part of the mantle cavity there are gills- one on both sides of the visceral mass.
The blood is set in motion three hearts – main consisting of a ventricle and two atria, and two gills. The main heart pumps blood throughout the body, and the rhythmic contractions of the gill hearts push venous blood through the gills, from where it, enriched with oxygen, enters the atrium of the main heart. The heart rate depends on the water temperature. For example, an octopus at a water temperature of 22 °C has a heart rate of 40–50 beats per minute.
Cephalopods have an almost closed circulatory system: in many places (skin, muscles) there are capillaries through which arteries turn into veins.
The blood of cephalopods is blue due to the presence of respiratory pigment. hemocyanin, containing copper. Hemocyanin is produced in special gill glands.
A highly developed circulatory system allows cephalopods to reach gigantic sizes.
The excretory organs are the kidney sacs, the appendages of the gill hearts, and the gills themselves. The main metabolic product of cephalopods is ammonia (more precisely, ammonium ions).
All cephalopods are dioecious. Male cephalopods are usually smaller than females, and when mature, one or two of their arms will change, turning into hectocotylus, with the help of which males transfer spermatophores to the female’s seminal receptacles during copulation.
Spermatophores - “packets” with seminal fluid - are complex and have different shapes in different species. Typically, the spermatophore is a thin, slightly curved tube, shaped like a Cossack saber. Spermatophores are formed in a special section associated with the testis - the spermatophore organ; they accumulate in a special storage facility - the spermatophore sac. During mating, spermatophores emerge through the excretory canal, are picked up by the hectocotylus and transferred to the female's spermatheca. In cuttlefish and some squids, the spermatheca is located on the oral membrane of the female.
The nervous system of cephalopods is more complex than that of other invertebrate animals. In terms of complexity, it is not inferior to the nervous system of fish. The ganglia are very close together and essentially form a single nervous mass - the brain, which in intrashell cephalopods is also enclosed in a cartilaginous capsule - the skull. In terms of relative mass, the brain of cephalopods exceeds that of fish, but is inferior to the brain of birds and mammals.
The brain consists of lobes, the total number of which in an octopus is 64. The optical lobes are the largest of them - they can account for 4/5 of the volume of the brain. In terms of subtlety of feelings, accuracy of perception and complexity of responses and behavior, cephalopods are superior to many marine animals. Cephalopods are characterized by good memory, and they distinguish between short-term, intermediate and long-term memory. Octopuses and cuttlefish learn well, and they solve some problems as successfully as rats.
Among the sense organs, the eyes have reached the greatest complexity and perfection. The eyes are usually located in the recesses of the cartilaginous head capsule and have a cornea, an iris with a pupil capable of contracting and dilating, a lens and a retina. There is even an eyelid, as in onychotheutid squids, which can cover the eye.
The eye of an octopus is almost no different from the eye of mammals and humans. But there are still differences between them. For example, the cornea of the eye in most cephalopods is not continuous, but is pierced in front by a small (in cuttlefish) or rather wide (in squid) hole. The lens of the eye in cephalopods is not elliptical, but round, divided in half by a thin epithelial plate. Accommodation of the eye (setting vision to different distances, focusing) in cephalopods is achieved by moving it away or bringing it closer to the retina.
None of the inhabitants of the sea have such keen eyes as cephalopods. Only the eyes of an owl, a cat, and a human can compete with them.
And in terms of eye size, cephalopods hold the record. The cuttlefish's eye is only ten times smaller than itself. The giant squid has an eye the size of a car headlight. In many deep-sea cephalopods, the eyes occupy most of the head.
The suction cups of the hands contain tactile and taste buds. Cephalopods recognize the taste of food mainly with their hands. There are a huge number of taste-sensitive cells on the rims of the suckers, so each sucker is involved in tasting food.
Cephalopods also have olfactory organs: in squids these are papillae, or papillae, located on the head below the eyes, and in octopuses these are olfactory pits.
In the occipital part of the cartilaginous skull of cephalopods there are two statocysts - organs of balance. These are a pair of bubbles filled with liquid and containing calcareous pebbles inside - statoliths. At the slightest change in body position, the statoliths touch sensitive cells and the walls of the vesicle, and the animal orients itself in space.
As for hearing, it is not yet clear whether cephalopods have it. They are believed to be deaf, and it has even been suggested that deafness is a special adaptation that protects these mollusks from shock that can be caused by the sonar of whales - their worst enemies.
Cephalopods live only in oceans and fully saline seas. The salt content in water must be at least 33 0. therefore, these mollusks are not found either in the Black or Baltic Seas. They are extremely numerous in tropical and subtropical waters, but also live in temperate waters and polar seas.
Cephalopods have been consumed as food for a long time. In China, Japan and Korea, the use of these animals as food goes back centuries; in Mediterranean countries it also has a very long history. Ancient writers, in particular Aristotle, Plutarch and Pliny, report that in ancient Greece and ancient Rome, skillfully prepared octopus was a common food. Cephalopods were also used in medicine and perfumery. Jewelry was made from beautiful nautilus shells, and paint and ink were made from the ink liquid of sepia cuttlefish.
And until now, in the countries of the Mediterranean and Southeast Asia, cephalopods are very popular. They are prepared in ice cream, fresh, dried and canned form.
Recently, interest in cephalopods has increased sharply. It is caused by the unfavorable state of fish stocks and the need to find additional biological resources that are not yet used by fisheries, which could make up for the deficiency of protein foods. Therefore, a whole complex of features puts cephalopods in the category of valuable commercial animals. In terms of basic nutritional indicators - calorie content and protein composition - squid and other cephalopods are superior to other edible shellfish and even some fish and are only slightly inferior to beef and veal.
To this should also be added a high yield of production - 80% of the mass of the mollusk is used for food. Moreover, even the insides of cephalopods are valuable, since they contain a large number of different substances from which potent medicines can be prepared. Thus, cephalopods can be used completely, 100%.
Control questions:
Which character traits Are the structures inherent in mollusks?
What classes is the phylum Mollusca divided into?
What are the main structural features characteristic of the class Gastropods?
What structure do bivalves have?
What structure do mollusks of the class Cephalopods have?
What is the significance of Mollusks in nature and human economic activity?
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