Class Gastropods- the most diverse and widespread group of mollusks.

There are about 90 thousand modern species of gastropods living in the seas (rapana, cones, murex), fresh water bodies (ponds, coils, meadows), as well as on land (slugs, grape snails).

External structure

Most gastropods have a spirally twisted shell. In some, the shell is underdeveloped or completely absent (for example, in naked slugs).

The body consists of three sections: heads, torso and legs.

On the head are one or two pairs of long soft tentacles and a pair of eyes.

The body contains internal organs.

The leg of gastropods is adapted for crawling and is a muscular outgrowth of the abdominal part of the body (hence the name of the class).

Common pondweed- lives in fresh water bodies and shallow rivers throughout Russia. It feeds on plant foods, scraping the soft tissues of plants with a grater.

Digestive system

In the oral cavity of gastropods there is a muscular tongue with chitinous teeth that form a “grater” (or radula). In herbivorous mollusks, the grater (radula) is used to scrape off plant food, in carnivorous mollusks it helps to retain prey.

The salivary glands usually open into the oral cavity.

The oral cavity passes into the pharynx, and then into the esophagus, which leads to the stomach and intestines. Channels flow into it digestive gland. Undigested food remains are thrown out through anal hole.

Nervous system

Nervous system ( shown in yellow in the figure) consists of several pairs of well-developed nerve nodes located in different parts of the body, and the nerves coming from them.

Gastropods have developed sensory organs, they are located mainly on the head: eyes, tentacles - organs of touch, organs of balance. Gastropods have well-developed olfactory organs - they can recognize odors.

Circulatory system

Gastropods have an open circulatory system consisting of a heart and blood vessels. The heart consists of two chambers: the ventricle and the atrium.

Respiration in mollusks living in water is carried out by gills, and in terrestrial ones - with the help of the lung.

In the mantle cavity, most aquatic gastropods have one or, less commonly, two gills.

In pond snails, coil snails, and grape snails, the mantle cavity acts as a lung. Oxygen from the atmospheric air filling the “lung” penetrates through the wall of the mantle into the blood vessels branched in it, and carbon dioxide from the blood vessels enters the cavity of the “lung” and goes out.

Excretory system

The excretory organs of mollusks are one or two kidneys.

Metabolic products that are unnecessary for the body come from the blood to the kidney, the duct from which opens into the mantle cavity.

The release of carbon dioxide from the blood and the enrichment of oxygen occurs in the respiratory organs (gills or lungs).

Reproduction

Shellfish breed only sexually.

Ponds, coils, slugs are hermaphrodites.

They usually lay fertilized eggs on plant leaves and various water objects or between lumps of soil. Small snails emerge from the eggs.

Many marine gastropods are dioecious animals; they develop from larval stage - swallowtail.

Meaning

Many shellfish serve as food for fish and birds. Terrestrial gastropods are eaten by amphibians, moles, and hedgehogs. Some species of gastropods are also eaten by humans.

Among the gastropods there are pests of gardens and vegetable gardens - slugs, grape snails, etc.

YouTube Video

Class Bivalve (elasmobranch) molluscs

Theory:

Bivalves Exclusively aquatic animals, they lead a mostly sedentary lifestyle. Most of them live in the seas (mussels, oysters, scallops), and only a small part lives in fresh water bodies (toothless, pearl barley, dreisena).

Characteristic feature of Bivalves - lack of head.

The shell of bivalve mollusks consists of two valves (hence the name of the class).

Representative - common toothless. Her body consists of a torso and legs covered with a mantle. It hangs from the sides in the form of two folds. The cavity between the folds and the body contains the leg and gill plates. The toothless fish, like all bivalves, has no head.

At the posterior end of the body, both folds of the mantle are pressed against each other, forming two siphons: lower (input) and upper (outlet). Through the lower siphon, water enters the mantle cavity and washes the gills, which ensures respiration.

Digestive system

Bivalve mollusks are characterized by a filtration feeding method. They have an inlet siphon, through which water with food particles suspended in it (protozoa, unicellular algae, remains of dead plants) enters the mantle cavity, where this suspension is filtered. Filtered food particles are directed to the mouth opening needle; then goes to esophagus, stomach, intestines and through anal hole enters the outlet siphon.
The toothless has a well developed digestive gland, the ducts of which flow into the stomach.

Bivalves breathe using gills.

Circulatory system

The circulatory system is not closed. It includes the heart and blood vessels.

Reproduction

Toothless is a dioecious animal. Fertilization occurs in the mantle cavity females, where sperm enter through the lower siphon along with water. Larvae develop from fertilized eggs in the gills of the mollusk.

Meaning

Bivalves are water filters, food for animals, used for human food (oysters, scallops, mussels), and producers of mother-of-pearl and natural pearls.

The shell of bivalve mollusks consists of three layers:

• thin outer - horny (organic);
• the thickest medium - porcelain-like (limestone);
• internal - mother-of-pearl.

The best varieties of mother-of-pearl are distinguished by the thick-walled shells of the sea pearl oyster, which lives in warm seas. When certain areas of the mantle are irritated by grains of sand or other objects, pearls form on the surface of the nacreous layer.

Shells and pearls are used to make jewelry, buttons, and other items.

Some mollusks, such as the shipworm, so named for its body shape, harm wooden structures in water.

YouTube Video

Class Cephalopods

Theory:

Cephalopods- a small group of highly organized animals, distinguished by the most perfect structure and complex behavior among other mollusks.

Their name - “Cephalopods” - is explained by the fact that the leg of these mollusks has turned into tentacles (usually 8-10 of them), located on the head around the mouth opening.

Cephalopods live in seas and oceans with a high salt content (they are not found in the Black, Azov and Caspian Seas, the water of which is desalinated by the rivers flowing into them).

Most cephalopods are free-swimming molluscs. Only a few live at the bottom.

Modern cephalopods include cuttlefish, squid, and octopuses. Their body sizes range from a few centimeters to 5 m, and the inhabitants of greater depths reach 13 m or more (with elongated tentacles).

External structure

The body of a cephalopod bilaterally symmetrical. It is usually divided by an interception into a body and a large head, and the leg is modified into a funnel located on the ventral side - a muscular conical tube (siphon) and long muscular tentacles with suction cups located around the mouth (octopuses have 8 tentacles, cuttlefish and squid have 10, nautilus have about 40). Swimming is aided by the pulsating ejection of water from the mantle cavity through the siphon - jet motion.

The body of most cephalopods lacks an external shell; there is only an underdeveloped internal shell. But octopuses don’t have shells at all. The disappearance of the shell is associated with the high speed of movement of these animals (the speed of some squids can exceed 50 km/h).

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 whose main feature 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 they can get the loot from a closed jar 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.

Sense organs. The best developed are the eyes, which are very complex formations. In boats, they have the shape of a glass with a very narrowed outer opening, lined with retina; There is no cornea, lens or vitreous body.

In inner shells, the eyes are much more complex; they have a lens, iris and cornea, and in terms of visual acuity and ability to accommodate, they can be comparable to the eyes of vertebrates.

In some forms, the eyes reach enormous sizes, for example, in giant squids (Architeutis) they reach 30 cm in diameter.

There are organs of smell (osphradia) and organs of balance (statocysts).

Digestive system. The digestive system begins with a mouth opening located on the head inside the crown of tentacles. In the oral cavity there is always a pair of powerful horny jaws that look like a bird's beak, and a radula, which is a thin chitinous plate with sharp teeth. One or two pairs of salivary glands empty into the oral cavity. Next comes the esophagus, which passes into the stomach. The ducts of the voluminous, sometimes complex liver open into the stomach. FromThe intestine extends from the stomach, forming a more or less complex loop that opens into the mantle cavity.

. I - dorsal plate; II - head cartilage

Thus, in cephalopods, as in gastropods, the anus lies not behind, but in front, and the digestive tract forms a sharp knee.

Argonauta argo . I—female, somewhat reduced; II -male, greatly enlarged:

1 - false shell - brood pouch, 2 - two tentacles forming it, 3 - hectocotyls inexpanded form

However, in cephalopods this is a consequence of a simple bending of a long visceral sac, but not a consequence of a torsion process, as in gastropods.

Not far from the anus, an ink gland flows into the intestines of many cephalopods, in which a black secretion is produced.

Respiratory system. The respiratory system is represented by two or four gills located in the mantle cavity. The circulation of water in it is ensured by the rhythmic contractions of the mantle itself during movement.

Circulatory system. It is very complex, almost closed, since a number of capillaries of the arteries directly pass into the capillaries of the veins. From the gills, oxidized blood is directed to the atria, from there to the stomach and then distributed throughout the body, pouring out in some places into the blood sinuses.

Excretory system. The excretory system is represented by altered metanephridia, in most cases connecting to the body cavityor its rudiment and opening into the mantle cavity. The boats have two pairs of them, the rest have one. There is a fairly close connection between the kidneys and the circulatory system (see above).

More interesting articles

Latin name Cephalopoda

Cephalopods General characteristics

The most highly organized animals among invertebrates. This is a relatively small group (about 730 species) of marine predators, the evolution of which is associated with the reduction of the shell. Only the most primitive four-gill mollusks have an external shell. The remaining bibranchial cephalopods, capable of rapid and prolonged movements, have only rudiments of shells that play the role of internal skeletal formations.

Cephalopods are usually large animals, their body length is at least 1 cm. Among deep-sea forms there are giants up to 18 m. Pelagic cephalopods (squid) have a streamlined body shape (similar to a rocket), they move the fastest. At the rear end of their body there are fins that stabilize movement. Benthic forms - octopuses - have a sac-like body, the front end of which forms a kind of parachute due to the fused bases of the tentacles.

External structure

The body of cephalopods consists of a head and a trunk. The leg, characteristic of all mollusks, is greatly modified. The back of the leg turned into a funnel - a conical tube leading into the mantle cavity. The funnel is located behind the head on the ventral side of the body. It is the organ with which mollusks swim. In a cephalopod of the genus Nautilus, which has retained many of the most ancient structural features of cephalopods, the funnel is formed by rolling a leaf-shaped leg into a tube, which has a regular wide sole. In this case, the wrapping edges of the leg do not grow together. Nautiluses use their legs to either slowly crawl along the bottom, or rise and swim slowly, carried by currents. In other cephalopods, the funnel blades are initially separate, but in adult animals they are fused into a single tube.

Around the mouth there are tentacles, or arms, which are seated with several rows of strong suckers and have powerful muscles. It turns out that the tentacles of cephalopods, like the funnel, are homologues of part of the leg. In embryonic development, the tentacles are laid on the ventral side behind the mouth from the leg bud, but then move forward and surround the mouth opening. The tentacles and infundibulum are innervated by the pedal ganglion. Most cephalopods have 8 (octopods) or 10 (decapods) tentacles; primitive mollusks of the genus Nautilus have up to 90. The tentacles are used to capture food and move; the latter is characteristic primarily of benthic octopods, which walk along the bottom on their tentacles. The suckers on the tentacles of many species are armed with chitinous hooks. In decapods (cuttlefish, squids), two of the ten tentacles are significantly longer than the others and are equipped with suckers at the extended ends. These are hunting tentacles.

Mantle and mantle cavity

The mantle covers the entire body of cephalopods; on the dorsal side it fuses with the body, on the ventral side it covers a large mantle cavity. The mantle cavity communicates with the external environment through a wide transverse slit located between the mantle and the body and running along the anterior edge of the mantle behind the funnel. The wall of the mantle is very muscular.

The structure of the muscular mantle and funnel is a device with the help of which cephalopods swim, and move the rear end of the body forward. This is a kind of “rocket” engine. In two places on the inner wall of the mantle at the base of the funnel there are cartilaginous projections called cufflinks. When the muscles of the mantle contract and press against the body, the front edge of the mantle, with the help of cufflinks, seems to be “fastened” to the recesses at the base of the funnel and the gap leading to the mantle cavity is closed. In this case, water is forcefully pushed out of the mantle cavity through the funnel. The body of the animal is thrown back some distance by the push. This is followed by relaxation of the muscles of the mantle, the cufflinks “unfasten” and water is absorbed through the mantle slit into the mantle cavity. The mantle contracts again and the body receives a new push. Thus, the rapidly successive alternating compression and stretching of the muscles of the mantle enable cephalopods to swim at high speed (squids). The same mechanism creates the circulation of water in the mantle cavity, which ensures respiration (gas exchange).

In the mantle cavity there are gills with the structure of typical ctenidia. Most cephalopods have one pair of ctenidia, and only the nautilus has 2 pairs. This is the basis for dividing the class of cephalopods into two subclasses: bibranchia (Dibranchia) and fourbranchia (Tetrabranchia). In addition, the anus, a pair of excretory openings, genital openings and openings of the nidamental glands open into the mantle cavity; in the nautilus the osphradia are also located in the mantle cavity.

Sink

Most modern cephalopods do not have shells at all (octopuses) or have vestigial shells. Only the nautilus has a well-developed thin shell. It should be borne in mind that the Nautilos genus is very ancient, having changed very little since the Paleozoic. The shell of the nautilus is twisted spirally (in a plane of symmetry) onto the head. Inside, it is divided into chambers by partitions, and the animal’s body is placed only in the front part, the largest chamber. A siphon extends from the back of the body of the nautilus, which passes through all the partitions to the top of the shell. Using this siphon, the chambers of the shell are filled with gas, which reduces the density of the animal.

Modern bibranchial cephalopods are characterized by an internal underdeveloped shell. The most complete spiral shell is preserved only in the small mollusk Spirula, which leads a bottom-dwelling lifestyle. In cuttlefish, the shell leaves a wide and thick porous calcareous plate lying on the dorsal side under the mantle. It has a supporting function. In squids, the shell is represented by a narrow dorsal chitinoid plate. Some octopuses have two conchiolin rods under the mantle. Many cephalopods have completely lost their shells. Shell rudiments play the role of skeletal formations.

In cephalopods, for the first time, an internal cartilaginous skeleton appears, which has protective and supporting functions. Dibranchs have a developed cartilaginous head capsule surrounding the central nervous system and statocysts, as well as cartilages of the bases of the tentacles, fins and mantle cufflinks. Quadribranchs have a single piece of cartilage that supports the nerve centers and the anterior end of the digestive system.

Digestive system

The mouth is located at the anterior end of the body and is always surrounded by a ring of tentacles. The mouth leads into a muscular throat. It is armed with powerful horny jaws, similar to the beak of a parrot. The radula is located at the back of the pharynx. The ducts of one or two pairs of salivary glands open into the pharynx, the secretion of which contains digestive enzymes.

The pharynx passes into a long narrow esophagus, which opens into a pouch-shaped stomach. In some species (for example, octopuses), the esophagus forms a lateral protrusion - a goiter. The stomach has a large blind appendage into which the ducts of the usually bilobed liver open. The small (endodermic) intestine departs from the stomach, which makes a loop, moving forward, and passes into the rectum. The rectum, or hind, intestine opens with the anus, or powder, in the mantle cavity.

The duct of the ink sac flows into the rectum in front of the powder. This piriform gland secretes an inky fluid that is expelled through the anus and creates a dark cloud in the water. The ink gland serves as a protective device that helps its owner hide from persecution.

Respiratory system

The gills, or ctenidia, of cephalopods are located symmetrically in the mantle cavity in one or two pairs. They have a feathery structure. The epithelium of the gills is devoid of cilia, and water circulation is ensured by rhythmic contractions of the muscles of the mantle.

Circulatory system

The heart of cephalopods usually consists of a ventricle and two atria, only the nautilus has four. Two aortas depart from the ventricle - the cephalic and abdominal, branching into a number of arteries. Cephalopods are characterized by a large development of arterial and venous vessels and capillaries, which pass into each other in the skin and muscles. The circulatory system becomes almost closed, the lacunae and sinuses are less extensive than in other mollusks. Blood from the organs is collected through the venous sinuses into the vena cava, which form blind protrusions that protrude into the walls of the kidneys. Before entering the ctenidium, the afferent gill vessels (venae cava) form muscular extensions, or venous hearts, which pulsate and promote the flow of blood to the gills. The enrichment of blood with oxygen occurs in the capillaries of the gills, from where arterial blood enters the atria.

The blood of cephalopods is blue because its respiratory pigment, hemocyanin, contains copper.

Secondary body cavity and excretory system

In cephalopods, as in other mollusks, the secondary body cavity, or coelom, is reduced. The most extensive coelom, containing the heart, stomach, part of the intestine and gonads, is found in primitive four-branched cephalopods. In decapod bibranchs, the coelom is more reduced and is represented by two separate sections - pericardial and genital; in eight-legged bibranchs, the pericardial coelom contracts even more and contains only the pericardial glands, and the heart lies outside the coelom.

The excretory organs are represented by two or four kidneys. They usually begin as funnels in the pericardial cavity (in some forms the kidneys lose contact with the pericardium) and open with excretory openings in the mantle cavity, on the sides of the powder. The kidneys are closely connected with blind protrusions of venous vessels, through which filtration and removal of metabolic products from the blood occurs. The pericardial glands also have an excretory function.

Nervous system

Dibranchial cephalopods are superior in terms of organization of the nervous system of all invertebrate animals. All ganglia characteristic of these mollusks come together and form the brain - a common nerve mass surrounding the beginning of the esophagus. Individual ganglia can only be distinguished through sections. There is a division of the paired pedal ganglia into the ganglia of the tentacles and the ganglia of the infundibulum. From the back of the brain, nerves arise that innervate the mantle and form two large stellate ganglia in its upper part. Sympathetic nerves that innervate the digestive system depart from the buccal ganglia.

In primitive quadribranchs, the nervous system is simpler. It is represented by three nerve semirings, or arches - suprapharyngeal and two subpharyngeal. Nerve cells are distributed evenly on them, without forming ganglion clusters. The structure of the nervous system of fourgills is very similar to that of chitons.

Sense organs

In cephalopods they are highly developed. Touch cells are located throughout the body, especially concentrated on the tentacles.

The olfactory organs of bibranchs are special olfactory pits, and only nautilus, i.e., quadribranchs, have osphradia.

All cephalopods have complex statocysts located in the cartilaginous capsule surrounding the brain.

The most important role in the life of cephalopods, especially in hunting for prey, is played by eyes, which are very large and of great complexity. The eyes of the nautilus are the simplest. They represent a deep eye fossa, the bottom of which is formed by the retina.

The eyes of bibranchial cephalopods are much more complex. The cuttlefish's eyes have a cornea, iris, lens, vitreous body and a very well developed retina. The following structural features of the cephalopod eye are noteworthy. 1. Many mollusks have a small hole in the cornea. 2. The iris also forms an opening - the pupil, leading into the anterior chamber of the eye. The pupil can contract and dilate. 3. A spherical lens, formed by two fused halves, is not capable of changing curvature. Accommodation is achieved with the help of special eye muscles that remove or bring the lens closer to the retina, as is done when focusing the lens of a photographic camera. 4. The retina consists of a huge number of visual elements (there are 105,000 visual cells per 1 mm 2 of the retina in the cuttlefish, and 162,000 in the squid).

The relative and absolute size of the eyes in cephalopods is larger than in other animals. Thus, the eyes of a cuttlefish are only 10 times smaller than the length of its body. The diameter of the eye of a giant octopus reaches 40 cm, and that of a deep-sea squid is about 30 cm.

Reproductive system and reproduction

All cephalopods are dioecious, and some have very pronounced sexual dimorphism. An extreme example in this regard is the wonderful eight-legged mollusk - the boat (Argonauta argo).

The female boat is relatively large (up to 20 cm) and has a shell of a special origin, not homologous to the shell of other mollusks. This shell is distinguished not by its mantle, but by its leg lobes. The shell is thin, almost transparent and spirally twisted. It serves as a brood chamber in which eggs are hatched. The male boat is many times smaller than the female and does not have a shell.

The gonads and reproductive ducts in most cephalopods are unpaired. Females are characterized by the presence of two or three paired and one unpaired nidamental glands, which secrete a substance from which the egg shell is formed. In males, sperm are enclosed in spermatophores of various shapes.

Of great interest is the method of fertilization in cephalopods. They do not actually mate. In sexually mature males, one of the tentacles is greatly modified; it turns into a hectocotylated tentacle, or hectocotylus. With the help of such a tentacle, the male removes spermatophores from his mantle cavity and transfers them to the mantle cavity of the female. In some cephalopods, especially in the boat (Argonauta) described above, the hectocotylated tentacle has a complex structure. After the tentacle is filled with spermatophores, it breaks off and swims on its own, and then climbs into the mantle cavity of the female, where fertilization occurs. Instead of the detached hectocotylus, a new one is regenerated.

Large eggs of cephalopods are laid in groups on various underwater objects (under stones, etc.). The eggs have a thick shell and are very rich in yolk. Crushing is incomplete, discoidal. Development is direct, without metamorphosis. A small mollusk, similar to an adult, emerges from the egg.

Classification

The class of cephalopods (Cephalopoda) is divided into two subclasses: 1. Tetrabranchia; 2. Dibranchia.

Subclass Tetrabranchia

This subclass is characterized by the presence of four gills and a large external shell, divided by partitions into many chambers. The subclass is divided into two orders: 1. Nautilids (Nautiloidea); 2. Ammonites (Ammonoidea).

Nautilids in the modern fauna are represented by only one genus - Nautilus, which includes several species. They have a very limited distribution in the southwest Pacific. Nautilids are characterized by many features of a more primitive structure: the presence of a shell, an unfused leg funnel, remnants of metamerism in the form of two pairs of gills, kidneys, atria, etc. Nautilids have survived little changed to our time since the Paleozoic. These living fossils are the remnants of a once rich fauna of fourgill cephalopods. Up to 2,500 fossil species of nautilids are known.

Ammonites are a completely extinct group of four-gill mollusks that also had a spirally twisted shell. Over 5,000 fossil species of ammonites are known. Remains of their shells are common in Mesozoic deposits.

Subclass Dibranchia

The subclass of bibranchs is characterized by an internal reduced shell (or lack thereof); Their respiratory organs are represented by two gills. The subclass is divided into two orders: 1. Decapoda (Decapoda); 2. Octopoda (Octopoda).

Order Decapoda

The most characteristic feature of decapods is the presence of 10 tentacles, of which 2 are hunting tentacles; many retain a rudimentary shell. Representatives include cuttlefish (Sepia officinalis), various species of fast-swimming squid from the genus Ommatostrephes (hundreds of them chasing schools of herring), from the genus Loligo, etc.

Decapods already existed in the Triassic, and they had an internal, but more developed shell. Often found in Mesozoic deposits, devil's fingers represent the remnant of the posterior part of the shell of Mesozoic decapod belemnites (Belemnoidea), pelagic animals that resembled squid in shape.

Order Octopoda (Octopoda)

Unlike decapods, they are predominantly benthic animals, with eight tentacles and lacking a shell. Representatives are different species of octopuses, as well as Argonauta and others.

The most important representatives of the class of cephalopods and their practical significance

Modern cephalopods are an essential part of the marine and oceanic fauna. They are distributed mainly in the southern seas and in seas with fairly high salinity. In Russia, the most cephalopods are in the Far Eastern seas. There are cephalopods in the Barents Sea. Cephalopods do not live in the Black and Baltic Seas due to the low salinity of these seas. Cephalopods are found at very different depths. Among them there are many deep-sea forms. Being predators, cephalopods feed on various marine animals: fish, crustaceans, mollusks, etc. Some of them cause great harm, destroying and spoiling schools of valuable commercial fish. Such, for example, are the Far Eastern squid Ommatostrephes sloani pacificus.

Among cephalopods there are very large forms, up to 3-4 m or more in size. The largest known cephalopod is the deep-sea squid (Architeuthis dux), a decapod. This real giant among cephalopods, and among invertebrates in general, reaches a length of 18 m, with a tentacle length of 10 m and a diameter of each tentacle of 20 cm. We know about such giants, unfortunately not yet caught alive, from their remains , found in the stomachs of killed toothed whales - sperm whales. Many toothed whales feed on cephalopods, as well as other sea predators: sharks, pinnipeds (seals), etc.

Cephalopods are also eaten by humans. Thus, cuttlefish and octopuses are eaten by the population of Mediterranean countries. In many countries, cuttlefish and squid are used as commercial fish.

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 on 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. In a burning light bulb, the same amount of energy is wasted 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 of 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 the 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 their 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 who is 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.