Cephalopods, the most highly organized mollusks, number about 650 species ranging in size from 1 cm to 5 m (and even up to 13 m - this is the body length of a giant squid). They live in seas and oceans, both in the water column and at the bottom. This group of mollusks includes octopuses, squids and cuttlefish (Fig. 81).

Rice. 81. Diversity of cephalopods: 1 - octopus; 2 - nautilus; 3 - squid; 4 - cuttlefish; 5 - Argonaut

These mollusks are called cephalopods because their legs have turned into tentacles, which are located in a corolla on the head, around the mouth opening.

External building. The body of cephalopods is 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 located around the mouth (Fig. 82). Octopuses have eight tentacles, cuttlefish and squids have ten. The inner side of the tentacles is lined with numerous large disc-shaped suckers.

Rice. 82. External appearance and internal structure of an octopus: 1 - horny jaws; 2 - brain; 3 - siphon; 4 - liver; 5 - pancreas; 6 - stomach; 7 - mantle; 8 - gonad; 9 - kidney; 10 - heart; 11 - gills: 12 - ink sac

The body is covered on all sides with a mantle. At the junction of the body and the head, the mantle cavity communicates with the external environment through a slit-like opening. Sea water is sucked into the mantle cavity through this gap. Then the gap is closed with special cartilaginous “cufflinks”. After this, water from the mantle cavity is forcefully pushed through the funnel, giving the animal a reverse push. Thus, cephalopods move with the rear end of the body forward in a reactive manner. The speed of some squids can exceed 50 km/h. Cuttlefish and squid have additional swimming organs - a pair of fins on the sides of the body.

Cephalopods are capable of quickly changing body color; deep-sea species have luminescent organs.

Internal skeleton. In most cephalopods, the shell is almost undeveloped (reduced) and hidden in the body of the animal. In cuttlefish, the shell looks like a calcareous plate lying under the integument on the dorsal side of the body. The squid has a small “feather” left from its shell, while octopuses have no shell at all. The disappearance of the shell is associated with the high speed of movement of these animals.

Cephalopods have a special internal skeleton formed by cartilage: the brain is protected by a cartilaginous skull, supporting cartilages are present at the base of the tentacles and fins.

Digestive system. The mouth opening (at the crown of the tentacles) is surrounded by two thick horny jaws of black or brown color, curved like the beak of a parrot. The tongue is located in the highly developed muscular pharynx. There is a grater on it, with which animals grind food. The ducts of the poisonous salivary glands flow into the pharynx. Next comes a long esophagus, a muscular pouch-like stomach and a long intestine that ends in the anus. A duct of a special gland, the ink sac, opens into the hindgut. In case of danger, the mollusk releases the contents of its ink sac into the water and, under the protection of this “smoke screen”, hides from the enemy.

All cephalopods are predators, attacking mainly fish and crustaceans, which they grab with their tentacles and kill with the bite of their jaws and the poison of the salivary glands. Some animals of this class eat mollusks, including cephalopods, carrion, and plankton.

Nervous system. In cephalopods it reaches a high level of complexity. The nerve ganglia of the central nervous system are very large and form a common peripharyngeal nerve mass - the brain. Two large nerves arise from its posterior section.

Sense organs well developed. In terms of structural complexity and visual acuity, the eyes of cephalopods are not inferior to the eyes of many vertebrates (Fig. 83). Among cephalopods there are especially large-eyed ones. The diameter of the giant squid's eye reaches 40 cm. Cephalopods have organs of chemical sense and balance; tactile, photosensitive and taste cells are scattered in the skin.

Rice. 83. Diagram of the structure of the eye of a cephalopod: 1 - refractive lens; 2 - layer of light-perceiving sensitive cells

Respiratory system. Most cephalopods have one pair of gills, which are located in the mantle cavity. Rhythmic contractions of the mantle serve to change water in the mantle cavity, ensuring gas exchange.

Circulatory system. In cephalopods it is almost closed - in many places the arteries, after releasing oxygen to the tissues, pass through the capillaries into veins. The heart consists of one ventricle and two atria. Large vessels depart from the heart, which are divided into arteries, and those, in turn, into a network of capillaries. The afferent vessels carry venous blood to the gills. Before entering the gills, the afferent vessels form muscular expansions, the so-called venous hearts, which, with their rhythmic contractions, contribute to the rapid flow of blood into the gills.

The number of heartbeats in cephalopods is 30-36 times per minute. Instead of hemoglobin, which contains iron, which causes the red color of blood in vertebrates and humans, the blood of cephalopods contains a substance that includes copper. Therefore, the blood of cephalopods is bluish in color.

Reproduction. Cephalopods are dioecious, and sexual dimorphism (differences in the size and external structure of the male and female) is pronounced in some species, for example in the argonaut (Fig. 84).

Rice. 84. Argonaut: A - female; B - male

Fertilization occurs in the mantle cavity of the female. One of the tentacles plays the role of a copulatory organ. The sperm of males are glued together into packets surrounded by a dense membrane - spermatophores.

The eggs of cephalopods are large, rich in yolk. There is no larval stage. A young mollusk emerges from the egg, its appearance resembling an adult animal. Female squids and cuttlefish attach eggs to underwater objects, and octopuses guard their clutches and young. Typically, cephalopods reproduce once in their lives, after which they die.

Humans use cephalopods: squid, octopus, and cuttlefish for food; From the secretion of the cuttlefish's ink sac he obtains sepia watercolor paint.

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

Exercises based on the material covered

1. Using Figure 81, characterize the features of the external structure and movement of cephalopods.
2. Name the distinctive features of the following organ systems of cephalopods: digestion, respiration, nervous, circulatory systems.
3. The structure of which organs confirms the higher level of organization of mollusks? Explain with examples.
4. What significance do representatives of cephalopods have in nature and human life?

Today class Cephalopods has about 700 species, which include exclusively inhabitants of reservoirs with high salt concentrations. Cephalopods are predators and are quite large in size. The body of cephalopods is divided into a head, a trunk and tentacles - a modified leg. Tentacles surround the mollusk's mouth. Typically, cephalopods have either 8 identical tentacles, or 8 long and 2 short.

Each of the tentacles has suckers, which help to capture, hold and deliver prey to the mouth. There are two large eyes on the head; only the nautilus- one of the tropical species of cephalopods.

External sink cephalopods do not, since in most of them it is an internal part of the body. U cuttlefish it is located under the skin. The nautilus has an external multi-chambered shell, one of the chambers of which contains the body, and the rest contain air. This allows the nautilus to move at a fairly high speed, as well as regulate the depth of its dive (filling the chambers with either water or air with a high nitrogen content). Cephalopods can move reactively, due to which the speed of some of them can be up to 70 km/h (squids).

Many cephalopods are able to change the coloring of their skin depending on the situation. It could be mimicry- camouflage to match the environment or a warning-threatening change of colors - bright, contrasting colors; quick change of colors. This feature of cephalopods is a consequence highly developed nervous system of mollusks, which consists of a complex brain, which is located in a primitive skull - a cartilaginous shell. The nervous system also consists of sense organs.

The complex nervous system of cephalopods caused them to appear conditioned reflexes and other signs of challenging behavior. For example, some mollusks of this class can produce an inky liquid, which, in times of danger, splashes out sharply, forming a black spot that prevents the mollusk’s enemy from seeing it. Thanks to this, the cephalopod can hide. There are also species that have salivary glands that secrete toxic substances. These substances help kill prey.

Reproductive system in cephalopods heterosexual. Development direct.

Many cephalopods are the target of industrial capture because they are used as food. Cuttlefish produce an inky liquid from which natural ink, sepia, is made. The undigested remains of cephalopods in the stomach of sperm whales form a special substance - ambergris, which is used in the perfume industry. Cephalopods are also the food source for most marine animals.

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 into the mouth opening pharynx; 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.

Reproduction

It includes the heart and blood vessels. Toothless is a dioecious animal. Fertilization occurs in the mantle cavity

Meaning

females, where sperm enter through the lower siphon along with water. Larvae develop from fertilized eggs in the gills of the mollusk.

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 a bivalve mollusk 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.

YouTube Video

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

Theory:

Class Cephalopods 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).

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).

Mollusks are so diverse that these animals rank second in number in the world, second only to arthropods. All three classes of these invertebrates share common features, for example, their body most often consists of three layers, while the body itself is enveloped in a skin “veil” called the mantle.

As a rule, these creatures, in addition to the body, have a leg and a head, but in different species some of these components may be absent. Let's discuss the most mobile class cephalopods. Unlike many of their counterparts, these animals spend most of their time in motion.

Moreover, they are quite fast, they can easily reach a speed of 50 kilometers per hour. Animals are capable of a complex chain of actions; they are the most “smart” among mollusks. Their home is the salty water of the oceans and seas. The dimensions are very varied, from one centimeter to several meters in length. Giant individuals can weigh almost half a ton.

Highly developed predatory creatures have the main distinguishing feature - their tentacles are located on the head, bordering the mouth. Only a few in this class have a sink; all the rest do without it.

There are more than seven hundred species of these invertebrates. Most likely, each of us has at least once seen a squid, even if not alive, or an octopus. Another popular and widely known representative of cephalopods is the cuttlefish.

The appearance of cephalopods is completely diverse. Their body may be like a rocket, a sac with several appendages, or a cap equipped with tentacles.

Inside the body there may be some kind of shell, but this is not at all the same calcareous “house” as in gastropods, for example. Thin plates, or even just lime needles - that’s what cephalopods replaced the shell.

TO features of cephalopods It can also be attributed to the fact that these invertebrates have a skeleton. But not in the usual sense, these are not bones. It consists of cartilage tissue. It protects the brain, secretes the eyeballs, and also extends to the base of the tentacles and fins.

Despite the fact that cephalopods are dioecious, they do not mate. When the male is ready for adulthood, one of his tentacle arms transforms in order to capture the reproductive cells in his mantle cavity and safely send them into the same cavity of the chosen female.

There is an even more interesting method of fertilization, inherent in other species: the selected tentacle of a male individual, filled with sperm, breaks away from the host’s body and goes into free swimming. Having discovered the female, this “love boat” enters her body. But the male does not remain crippled; a new one grows in place of the lost leg.

These predators lay their eggs in special. depressions at the bottom. Before the birth of their young, certain types of mollusks guard their offspring, but we are only talking about mothers. While protecting the clutch, the animal can become so weak that when the time comes for the babies to leave the “shell”, their mother dies from impotence.

The structure of cephalopods

Outside:

Mollusks are characterized by symmetry. Their body is the same on the right and left sides.

You will not find legs in these mollusks, like, for example, in snails. This is because it has transformed into a tube at the base of the body on the lower side. This siphon helps the animal to move quickly, the water accumulated inside is abruptly released from it and a reactive movement is created. Another appendage of the leg is the tentacles, there are either 8 or 10 of them.

The mantle, or fold of skin that surrounds cephalopod body. From above it adhered to the outer integument, but not from below, due to which a mantle cavity was formed. There is a narrow hole in the fold so that water can get in.

The mantle cavity is filled not only in order to be able to move, releasing water sharply through the crow (siphon), but also in order to breathe. After all, the gills are located there. As a rule, there are two of them, sometimes four. And the anus and genital opening also exit there.

The very strong tentacles of cephalopods are literally strewn with dozens of suckers. These prehensile toes initially originate in the leg buds. As the individual grows, they move forward and frame the mouth.

The tentacles serve not only as legs (i.e. for movement), but also as arms capable of grasping prey. But the brain sends certain signals to the limbs infrequently. In most cases, they simply move chaotically, succumbing to the influence of nerve cells.

Inside:

If in representatives of other classes of mollusks blood flows freely throughout the body, washing the organs, then circulatory system of cephalopods- closed. But the blood itself does not have a scarlet color; it is, one might say, colorless. The reason is simple - it does not contain hemoglobin.

Hemocyanin took its place (it contains traces of copper). As a result, the invertebrate became “blue blood”, i.e. When wounded, the blood turns into a bluish liquid. The structure of the heart is as follows: one ventricle, two atria (in rare cases - 4).

It knocks at a speed of three dozen times per minute. The mollusk is unique in that it has two more hearts, gills. They are needed to drive blood through the respiratory organs and supply them with oxygen.

Deserves special attention and nervous system of cephalopods. Animals can be called very inventive. The knots of nerves intertwine to form a decent-sized brain. As we already said, it is even surrounded by some kind of skull.

This is where the incredible abilities of cephalopods come from. Octopuses are most famous for them. Firstly, these creatures can be said to be trainable. They perfectly remember the sequence of actions necessary to complete a task in each specific case.

For example, they can open a container to get the desired item. If an individual understands that it cannot cope alone, it can attract its relatives. Together they develop entire hunting schemes.

By the way, the rectum of these tentacle owners has a very interesting feature - there is a special sac there. This bottle has two compartments. In the bottom there are spare grains of a special dye, in the top there is ready-made ink in case of need.

And you need this bluish-violet (sometimes black, brown) liquid in order to protect yourself in case of danger. Such a colored curtain disorients the enemy. A dark veil literally covers the water for several meters in the area. After ejection, this “weapon” is restored quite quickly; for some, half an hour is enough to be in full combat readiness.

It is also interesting that some researchers have noticed the similarity of these ink emissions with their owners in outline. Those. the animal leaves such a decoy for the enemy, and while he is trying to eat it, he can “run away.” In addition, the unique ink can deprive a number of predatory fish of their sense of smell.

And it will take them at least an hour to regain their sense of smell. These dyes are also unsafe for the mollusks themselves. Therefore, animals hastily leave the place where their “cloud” was released. As for human health, everything is calm here, ink will not harm us. Even if it gets into your eyes. Moreover, gourmets enjoy eating them.

These sea creatures touch with their whole body. Among other things, these mollusks have an excellent sense of smell, taste, and also excellent vision. They are characterized by very good eyesight. The eyes are usually large.

Kinds

• Fourgills

The most simply organized order of cephalopods. In addition to four gills, they have the same number of kidneys and atria. Among other things, their striking difference is the outer shell, which covers almost the entire body. They appeared on our planet about five hundred million years ago. Only one representative of these soft-bodied animals has survived to this day - nautilus.

The brown and white nautilus shell has a spiral curl. The inside is covered with mother-of-pearl. It contains several compartments. One of them serves as storage for the animal's body. The remaining cameras are needed for diving. If an invertebrate needs to reach the surface of the sea, it fills these containers with air, but if it needs to fall to the bottom, the air is replaced by water. Over the course of life, the number of compartments increases.

The cephalopod does not favor very great depths; it prefers not to descend below one hundred meters. This is because the shell is quite fragile, and the weight of the water can simply break it.

If we consider structure of cephalopods, then the nautilus has a more simplified configuration than its counterparts. Only part of the head and tentacles stick out from the animal’s “house”; it has as many as ninety of them. Like many other cephalopods, these processes have suckers; the “arms” themselves are quite muscular, which allows the individual to move and capture prey without any problems. Both animal and plant foods are consumed.

In addition, the head contains eyes and a mouth. Females are slightly smaller than males. This invertebrate has a well-developed sense of smell, but its vision is not so acute. The mantle, like a blanket, envelops the entire nautilus. By contracting this organ. The animal sharply pushes water out of it, thus moving in the water column.

As for reproduction, they become sexually mature when they reach approximately 10 centimeters in shell diameter (in general, an animal can grow a shell up to 25 cm in diameter). The male then places his reproductive cells into the female’s body. After six months, small nautiluses hatch from the laid eggs, completely repeating the structure of their parents.

In recent years, the population of these individuals has been declining. The reason is people's increased interest. After all, the animal shell is used as a decorative decoration. Keeping an invertebrate in captivity is quite expensive, and the individual itself will cost anyone who wants to purchase it a considerable amount.

• Dibranchs

As the name suggests, these animals have two gills. They are more complex than the representatives of the previous detachment. They do not have a shell in its classical sense. Only small inclusions inside the body are what is left of it. Their visual organs are quite developed.

The detachment is divided into two suborders:

1. Ten-armed (they have five pairs of tentacles, one of which is longer and serves as prehensile fingers).

Squid.

People know about three hundred species of such cephalopods. Most often, this animal looks like a long rocket with tentacles. By the way, they do not grow together; there are no membranes between them. But squids have outgrowths similar to fins. These two wings can reach quite large sizes and serve as soft-bodied wings for movement in water.

Like other species of cephalopods, reactive force also helps them move, and they can quickly change the direction of movement using a siphon. Thanks to the ability to control it, the animal can reverse and even fly above the surface of the water.

In a calm state, invertebrates do not look very impressive, their body is translucent, smooth, pinkish, or white, but they have the ability to phosphorescent with bright bluish colors. Squids acquired this ability thanks to specific bacteria located in their body. Thanks to its attractive glow, the squid attracts its prey.

The smallest individuals are 10 cm long, while the largest ones can grow up to one meter. There have long been legends about sea monsters attacking the ships of sailors. But then it became clear that these were just giant squids that reached a size of 18 meters, and one of their eyes was larger than a large watermelon. These individuals have a very interesting feature: their brain has a hole through which the esophagus passes. The animal's jaws are so powerful that they can easily bite through the bones of even the smallest fish.

Animals are quite intelligent, and have a brain surrounded by a kind of skull. The body is a mantle, with a chitinous substance inside (the shell took this form, the need for which was no longer necessary for the animal) and cephalopod organs.

Among these individuals there is also a very unusual fellow called a vampire. This species is considered a cross between octopuses and squids. Only its tentacles are connected by membranes along almost the entire length, and the body color is bright red.

Animals settle both in the dark depths of the sea and in shallow water (small individuals prefer such a home). They do not stay in one place for long and are constantly on the move. In just one day they can cover about 30 kilometers.

The squid's diet includes fish, other shellfish, and even smaller representatives of its species.

Animals have offspring only once a year. The female lays eggs, and the male hands her his reproductive cells in something like a bag. After which the larvae are born. They will be ready to give birth to their own offspring within a year or two. By the end of the third year of life, the animal dies.

The life of squids is not “sugar”. Because everyone who is not too lazy hunts them - from people to dolphins and birds. Their ability to move quickly and the presence of ink help soft-bodied animals not to turn into someone else's prey. By throwing them into the water, they confuse the enemy.

Among the squids, the following are very interesting: piglet squid (very small and looks like a pig’s face), glass squid (transparent, like glass, only the eyes and digestive organs stand out)

Cuttlefish.

The animal is not very large, its length can be only a couple of centimeters, or maybe 30. They do not live long, up to 2 years. They don’t really like company; most often they spend time alone, without particularly running from place to place. This rule is broken only when it’s time to reproduce.

These invertebrates even have some kind of mating game. True, immediately after fertilization of the eggs, adults can depart to another world. Unlike many mollusks, the cuttlefish goes hunting before dark, but if it itself risks becoming prey, it buries itself in the sand, using its fins for this.

In appearance, the cuttlefish's body resembles a flattened cylinder. Inside it there is some kind of bone - a transformed shell. This plate not only serves as a shield for the internal organs, running across the entire back, but also helps control the speed of movement of the animal by filling the compartments into which it is divided with water. Regarding nervous cephalopod systems, then it is much more developed than in other representatives of the species.

On the head of the cuttlefish there are huge eyes and a special outgrowth with which it catches and grinds food. If the animal is not in danger, its arms are pressed tightly against each other and extended, and a pair of tentacles are folded into a special shape. compartments

The cuttlefish does not like to stay in one color for a long time; it easily changes its shades. These can be completely different patterns. For example, the one called the striped one is deadly poisonous. Despite this, different types of shellfish are used as food for people.

1. Eight-armed

They have four pairs of arms, and at the base they are specially connected. film - membrane. Otherwise, everything is the same as in other cephalopods - the mantle sac (body) is soft and shapeless if it lands on land.

Octopus.

The eyes are large and sit on protrusions. Moreover, if necessary, they can be easily moved and focused on a specific object. There are a lot of suckers on the tentacles (they can go in three rows, and the number reaches up to 2 thousand), they are capable of sending signals about the taste of food. In addition, they often serve as legs; by moving them, the octopus literally slides along the bottom.

The covers of octopuses are usually burgundy-red. True, a little bit may change. Thanks to special cells, the mollusk can merge with its surroundings. The octopus's favorite delicacy is crabs, fish, and lobsters. Their beak, similar to that of parrots, helps them absorb all this. The largest species weigh fifty kilograms.

If you notice a bright yellow specimen with blue circles on its skin while diving, it is better to leave quickly. After all, in front of you is a blue-ringed octopus. Its poison is fatal to us, and such a meeting can be fatal for a person.

Reproduction serves as the beginning of the life of the young and the end for their parents. The male dies as soon as he transfers it to the female with the help of special equipment. tubes of their sperm. She, in turn, will carry them within herself until the right time, until she decides to fertilize the eggs. There are often thousands of these eggs. After waiting for the little octopuses to hatch (this can take up to six months), the mother also leaves for another world.

Octopuses use cracks in rocks, holes and nests as a home, which cephalopods can easily build, because they are very smart. Their home is always clean. They are helped to do the cleaning by a stream of water, which is released sharply and cleans out all the debris with its flow. Animals try to get food at night. They are sleeping. By the way, with your eyes open.

Nutrition

When the mollusk notices the prey, it grabs it with its tentacles and drags it into its mouth. Poison is often used; it is secreted by the salivary gland. As a result, the prey dies. In the mouth opening there is something similar to a bird's beak (the animal uses it to injure the victim, immobilizing it and biting off pieces). This is the appearance of the jaws of invertebrates.

However, large fish are too tough for them. To get food inside, the animal grinds it with its radula (which looks like a tongue with small teeth), which is located in the pharynx. And then everything is standard: the esophagus, after which food goes into the stomach, ending its path with the anus. This is digestive system of cephalopods.

The diet of these creatures includes all kinds of fish, crustaceans, etc. It is worth noting that they do not disdain individuals of their own kind, eating them. And the strangest thing is that the same octopuses can eat their own body. True, after such a procedure the animal inevitably dies.

Meaning

What is it the meaning of cephalopods? Despite their considerable size, cephalopods themselves often become prey for other living creatures. They are included in the diet of dolphins. They become a delicacy for killer whales and sperm whales.

People also value the meat of cephalopods. This is because it is very rich in protein, but you won’t find fat in it. Mining takes place in five hundred countries around the world. They especially love to taste this delicacy in Thailand, Italy and Japan. China is not inferior to its neighbors either.

They are eaten raw, boiled, dried, canned and more. Every year, as many as a million tons of cephalopods are caught from the depths of the sea. Nets are used for mining. The best catch is usually in spring and early summer.

A special way of “fishing” is popular in the land of the rising sun. Clay jugs serve as traps; I tie a rope to them and throw them to the bottom. The mollusks climb there and feel very comfortable there, therefore, even when they try to take them out of the water, they are in no hurry to leave the shelter.

In addition to nutritional value, shellfish also have artistic value. Their ink produces not only watercolors, but also ink. People also use caught octopus as bait. They use it to catch fish.

And now about how these invertebrates can harm. Several cases of octopus invasion have been recorded in history. A sharp increase in their numbers led to the fact that hundreds of corpses of these animals ended up on the shore, due to a storm or low tide.

As a result, rotting bodies contaminated the soil and air. In addition, too many octopuses lead to the fact that the animals included in their diet are on the verge of extermination. We're talking about lobsters and crabs.

Class Cephalopoda

Cephalopods are the most highly organized mollusks. They are rightly called the “primates” of the sea among invertebrate animals for the perfection of their adaptations to life in the marine environment and the complexity of their behavior. These are mainly large predatory marine animals capable of actively swimming in the water column. These include squids, octopuses, cuttlefish, and nautiluses (Fig. 234). Their body consists of a torso and a head, and the leg is transformed into tentacles located on the head around the mouth, and a special motor funnel on the ventral side of the body (Fig. 234, A). This is where the name comes from - cephalopods. It has been proven that some of the tentacles of cephalopods are formed due to the cephalic appendages.

Most modern cephalopods have no or vestigial shells. Only the genus Nautilus has a spirally twisted shell, divided into chambers (Fig. 235).

Modern cephalopods include only 650 species, while fossil species number about 11 thousand. This is an ancient group of mollusks known since the Cambrian. Extinct species of cephalopods were predominantly testate and had an external or internal shell (Fig. 236).

Cephalopods are characterized by many progressive organizational features due to the active lifestyle of marine predators. At the same time, they retain some primitive features that indicate their ancient origin.

External structure. The features of the external structure of cephalopods are varied due to different lifestyles. Their sizes range from a few centimeters to 18 m in some squids. Nektonic cephalopods are usually torpedo-shaped (most squids), benthic ones have a sac-shaped body (many octopuses), and nektobenthic ones are flattened (cuttlefish). Planktonic species are small in size and have a gelatinous buoyant body. The body shape of planktonic cephalopods can be narrow or jellyfish-like, and sometimes spherical (squid, octopus). Benthopelagic cephalopods have a shell divided into chambers.

The body of cephalopods consists of a head and a trunk. The leg is modified into tentacles and a funnel. On the head there is a mouth surrounded by tentacles and large eyes. The tentacles are formed by the head appendages and the leg. These are food capture organs. The primitive cephalopod (Nautilus) has an indefinite number of tentacles (about 90); they are smooth, worm-shaped. In higher cephalopods, the tentacles are long, with powerful muscles and bear large suckers on the inner surface. The number of tentacles is 8-10. Cephalopods with 10 tentacles have two tentacles - hunting ones, longer, with suckers at the expanded ends,

Rice. 234. Cephalopods: A - nautilus Nautilus, B - octopus Benthoctopus; 1 - tentacles, 2 - funnel, 3 - hood, 4 - eye

Rice. 235. Nautilus Nautilus pompilius with a sawn shell (according to Owen): 1 - head hood, 2 - tentacles, 3 - funnel, 4 - eye, 5 - mantle, 6 - internal sac, 7 - chambers, 8 - partition between shell chambers, 9 - siphon

Rice. 236. Scheme of the structure of cephalopod shells in a sagittal section (from Gescheler): A - Sepia, B - Belosepia, C - Belemnites, D - Spirulirostra, E - Spirula, F - Ostracoteuthis, G - Ommastrephes, H - Loligopsis (C, D, E - fossils); 1 - proostracum, 2 - dorsal edge of the siphonal tube, 3 - ventral edge of the siphonal tube, 4 - set of phragmocone chambers, 5 - rostrum, 6 - siphon cavity

Rice. 237. Mantle cavity of cuttlefish - Sepia (according to Pfurscheller): 1 - short tentacles, 2 - hunting tentacles, 3 - mouth, 4 - opening of the funnel, 5 - funnel, 6 - cartilaginous pits of cufflinks, 7 - anus, 8 - renal papillae, 9 - genital papilla, 10 - gills, 11 - fin, 72 - cut line of the mantle, 13 - mantle, 14 - cartilaginous tubercles of cufflinks, 15 - pallial ganglion

and the remaining eight tentacles are shorter (squid, cuttlefish). Octopuses that live on the seabed have eight tentacles of equal length. They serve the octopus not only to capture food, but also to move along the bottom. In male octopuses, one tentacle is modified into a sexual one (hectocotyl) and serves to transfer reproductive products into the mantle cavity of the female.

The funnel is a derivative of the leg in cephalopods and serves for a “reactive” method of movement. Through the funnel, water is forcefully pushed out of the mollusk's mantle cavity, and its body moves reactively in the opposite direction. In the boat, the funnel is not fused on the ventral side and resembles the sole of the foot of crawling mollusks rolled into a tube. Evidence that the tentacles and funnel of cephalopods are derived legs is their innervation from the pedal ganglia and the embryonic anlage of these organs on the ventral side of the embryo. But, as already noted, some of the tentacles of cephalopods are derivatives of the cephalic appendages.

The mantle on the ventral side forms a kind of pocket - a mantle cavity that opens outwards with a transverse slit (Fig. 237). A funnel protrudes from this gap. On the inner surface of the mantle there are cartilaginous protrusions - cufflinks, which fit tightly into the cartilaginous grooves on the body of the mollusk, and the mantle is, as it were, fastened to the body.

The mantle cavity and the funnel together provide jet propulsion. When the muscles of the mantle relax, water enters through the gap into the mantle cavity, and when it contracts, the cavity is closed with cufflinks and the water is pushed out through the funnel. The funnel can bend to the right, left and even backward, which provides different directions of movement. The role of the steering wheel is additionally performed by the tentacles and fins - skin folds of the body. The types of movement in cephalopods are varied. Octopuses often move on tentacles and swim less often. In cuttlefish, in addition to the funnel, a circular fin serves for movement. Some umbrella-shaped deep-sea octopuses have a membrane between the tentacles - the umbrella - and can move due to its contractions, like jellyfish.

The shell of modern cephalopods is vestigial or absent. The ancient extinct cephalopods had a well-developed shell. Only one modern genus, Nautilus, has retained a developed shell. The shell of Nautilus, even in fossil forms, has significant morphofunctional features, in contrast to the shells of other mollusks. This is not only a protective device, but also a hydrostatic device. The nautilus has a spirally twisted shell divided into chambers by partitions. The body of the mollusk is placed only in the last chamber, which opens with its mouth outward. The remaining chambers are filled with gas and chamber liquid, which ensures the buoyancy of the mollusk’s body. Through

The siphon, the posterior process of the body, passes through the holes in the partitions between the chambers of the shell. Siphon cells are capable of releasing gases. When floating, the mollusk releases gases, displacing the chamber liquid from the chambers; when sinking to the bottom, the mollusk fills the chambers of the shell with chamber liquid. The propeller of the nautilus is a funnel, and the shell keeps its body suspended in the water. Fossil nautilids had a shell similar to that of the modern nautilus. The completely extinct cephalopods - ammonites also had an external, spirally twisted shell with chambers, but their partitions between the chambers had a wavy structure, which increased the strength of the shell. That is why ammonites could reach very large sizes, up to 2 m in diameter. Another group of extinct cephalopods, the belemnites (Belemnoidea), had an internal shell, overgrown with skin. Belemnites in appearance resembled shellless squids, but their body contained a conical shell divided into chambers. The top of the shell ended with a point - the rostrum. Belemnite shell rostrums are often found in Cretaceous deposits and are called "devil's fingers". Some modern shellless cephalopods have rudiments of an internal shell. Thus, on the cuttlefish’s back, under the skin, a calcareous plate is preserved, which has a chamber structure when cut (238, B). Only the Spirula has a fully developed spirally twisted shell under its skin (Fig. 238, A), and the squid has only a horny plate under its skin. Females of modern cephalopods, Argonauta, have a developed brood chamber resembling a spiral shell in shape. But this is only a superficial resemblance. The brood chamber is secreted by the epithelium of the tentacles, is very thin and is designed to protect the developing eggs.

Veils. The skin is composed of a single layer of epithelium and a layer of connective tissue. The skin contains pigment cells - chromatophores. Cephalopods are characterized by the ability to quickly change color. This mechanism is controlled by the nervous system and is carried out by changing the shape

Rice. 238. Shell rudiments in cephalopods (according to Natalie and Dogel): A - spirula; 1 - funnel, 2 - mantle cavity, 3 - anus, 4 - excretory opening, 5 - luminescent organ, 6 - fin, 7 - shell, 8 - siphon; B - Sepia shell; 1 - septa, 2 - lateral edge, 3 - siphonal fossa, 4 - rostrum, 5 - siphon rudiment, 6 - posterior edge of the proostracum

pigment cells. So, for example, a cuttlefish, swimming over sandy soil, takes on a light color, and over rocky soil - dark. .At the same time, in her skin, pigment cells with dark and light pigment alternately shrink and expand. If you cut the optic nerves of a mollusk, it loses the ability to change color. Due to the connective tissue of the skin, cartilage is formed: in cufflinks, the bases of the tentacles, around the brain.

Protective devices. Cephalopods, having lost their shells during the process of evolution, acquired other protective devices. Firstly, fast movement saves many of them from predators. In addition, they can defend themselves with tentacles and a “beak”, which is modified jaws. Large squids and octopuses can fight with large marine animals, such as sperm whales. Sedentary and small forms have developed protective coloration and the ability to quickly change color. Finally, some cephalopods, such as the cuttlefish, have an ink sac, the duct of which opens into the hindgut. Spraying the ink liquid into the water creates a kind of smoke screen, allowing the mollusk to hide from predators to a safe place. Cuttlefish ink gland pigment is used to make high-quality artist's ink.

Internal structure of cephalopods

Digestive system cephalopods bear the features of specialization in feeding on animal food (Fig. 239). Their food consists mainly of fish, crabs and bivalves. They grab prey with their tentacles and kill them with their jaws and poison. Despite their large size, cephalopods can only feed on liquid food, since they have a very narrow esophagus, which passes through the brain, enclosed in a cartilaginous capsule. Cephalopods have devices for grinding food. To chew prey, they use hard horny jaws, similar to the beak of a parrot. In the pharynx, food is ground by the radula and abundantly moistened with saliva. The ducts of 1-2 pairs of salivary glands flow into the pharynx, which secrete enzymes that break down proteins and polysaccharides. The second posterior pair of salivary glands secretes poison. Liquid food from the pharynx passes through the narrow esophagus into the endodermal stomach, into which the ducts of the paired liver flow, which produces a variety of digestive enzymes. The hepatic ducts are lined with small accessory glands, the collection of which is called the pancreas. The enzymes of this gland act on polysaccharides,

and therefore this gland is functionally different from the mammalian pancreas. The stomach of cephalopods usually has a blind sac-like process, which increases its volume, which allows them to absorb a large portion of food. Like other carnivorous animals, they eat a lot and relatively rarely. The small midgut departs from the stomach, which then passes into the posterior intestine, which opens through the anus into the mantle cavity. The duct of the ink gland flows into the hindgut of many cephalopods, the secretion of which has a protective significance.

Nervous system Cephalopods are the most highly developed among mollusks. The nerve ganglia form a large peripharyngeal cluster - the brain (Fig. 240), enclosed in a cartilaginous capsule. There are additional ganglia. The brain primarily consists of: a pair of large cerebral ganglia that innervate the head, and a pair of visceral ganglia that send nerve cords to the internal organs. On the sides of the cerebral ganglia there are additional large optic ganglia that innervate the eyes. From the visceral ganglia, long nerves extend to two star-shaped pallial ganglia, which develop in cephalopods in connection with the function of the mantle in their reactive mode of movement. The brain of cephalopods includes, in addition to the cerebral and visceral, pedal ganglia, which are divided into paired ganglia of the tentacles (brachial) and funnels (infidibular). A primitive nervous system, similar to the scalene system of bokonervna and monoplacophorans, is preserved only in Nautilus. It is represented by nerve cords forming the peripharyngeal ring without ganglia and the pedal arch. Nerve cords are covered with nerve cells. This structure of the nervous system indicates the ancient origin of cephalopods from primitive shelled mollusks.

Sense organs cephalopods are well developed. Their eyes, which are most important for orientation in space and hunting for prey, reach a particularly complex development. In Nautilus, the eyes have a simple structure in the form of a deep optic fossa (Fig. 241, A), while in other cephalopods the eyes are complex - in the shape of an optic vesicle and reminiscent of the structure of the eye in mammals. This is an interesting example of convergence between invertebrates and vertebrates. Figure 241, B shows the eye of a cuttlefish. The top of the eyeball is covered with the cornea, which has an opening into the anterior chamber of the eye. The connection of the anterior cavity of the eye with the external environment protects the eyes of cephalopods from the effects of high pressure at great depths. The iris forms an opening - the pupil. Light through the pupil hits the spherical lens formed by the epithelial body - the upper layer of the eye bladder. Accommodation of the eye in cephalopods occurs differently,

Rice. 239. Digestive system of the cuttlefish Sepia officinalis (according to Reseler and Lamprecht): 1 - pharynx, 2 - common salivary duct, 3 - salivary ducts, 4 - posterior salivary gland, 5 - esophagus, 6 - cephalic aorta, 7 - liver, 8 - pancreas, 9 - stomach, 10 - blind sac of the stomach, 11 - small intestine, 12 - hepatic duct, 13 - rectum, 14 - ink sac duct, 15 - anus, 16 - head cartilaginous capsule (cut), 17 - statocyst , 18 - nerve ring (cut)

Rice. 240. Nervous system of cephalopods: 1 - brain, 2 - optic ganglia, 3 - pallial ganglia, 4 - intestinal ganglion, 5 - nerve cords in the tentacles

Rice. 241. Eyes of cephalopods: A - Nautilus, B - Sepia (according to Hensen); 1 - cavity of the eye fossa, 2 - retina, 3 - optic nerves, 4 - cornea, 5 - lens, 6 - anterior chamber of the eye, 7 - iris, 8 - ciliary muscle, 9 - vitreous body, 10 - ocular processes of the cartilaginous capsule, 11 - optic ganglion, 12 - sclera, 13 - openings of the eye chamber, 14 - epithelial body

than in mammals: not by changing the curvature of the lens, but by bringing it closer to or away from the retina (similar to focusing a camera). Special ciliary muscles approach the lens, causing it to move. The cavity of the eyeball is filled with a vitreous body that has a light-refracting function. The bottom of the eye is lined with visual - retinal and pigment - cells. This is the retina of the eye. A short optic nerve departs from it to the optic ganglion. The eyes, together with the optic ganglia, are surrounded by a cartilaginous capsule. Deep-sea cephalopods have luminous organs on their bodies, built like eyes.

Organs of balance- statocysts are located in the cartilaginous capsule of the brain. The olfactory organs are represented by olfactory pits under the eyes or osphradia typical of mollusks at the base of the gills - in the nautilus. The taste organs are concentrated on the inner side of the ends of the tentacles. Octopuses, for example, use their tentacles to distinguish edible from inedible objects. The skin of cephalopods contains many tactile and light-sensitive cells. In search of prey, they are guided by a combination of visual, tactile and gustatory sensations.

Respiratory system represented by ctenidia. Most modern cephalopods have two, but Nautilus has four. They are located in the mantle cavity on the sides of the body. The flow of water in the mantle cavity, which ensures gas exchange, is determined by the rhythmic contraction of the muscles of the mantle and the function of the funnel through which water is pushed out. During the reactive mode of movement, the flow of water in the mantle cavity accelerates, and the intensity of respiration increases.

Circulatory system cephalopods are almost closed (Fig. 242). Due to active movement, their coelom and blood vessels are well developed and, accordingly, parenchymality is poorly expressed. Unlike other mollusks, they do not suffer from hypokenia - weak mobility. The speed of blood movement in them is ensured by the work of a well-developed heart, consisting of a ventricle and two (or four - in Nautilus) atria, as well as pulsating sections of blood vessels. The heart is surrounded by a large pericardial cavity,

Rice. 242. Circulatory system of cephalopods (from Abrikosov): 1 - heart, 2 - aorta, 3, 4 - veins, 5 - gill vessels, 6 - gill hearts, 7, 8 - renal portal system, 9 - gill veins

which performs many of the functions of the coelom. The cephalic aorta extends forward from the ventricle of the heart and the splanchnic aorta extends backward. The cephalic aorta branches into arteries that supply blood to the head and tentacles. Vessels extend from the splanchnic aorta to the internal organs. Blood from the head and internal organs is collected in the vena cava, located longitudinally in the lower part of the body. The vena cava is divided into two (or four in Nautilus) afferent gill vessels, which form contractile extensions - gill “hearts”, facilitating gill circulation. The afferent gill vessels lie close to the kidneys, forming small blind invaginations into the kidney tissue, which helps to free venous blood from metabolic products. In the gill capillaries, blood is oxidized, which then enters the efferent gill vessels, which flow into the atria. Some of the blood from the capillaries of the veins and arteries flows into small lacunae, and therefore the circulatory system of cephalopods should be considered almost closed. The blood of cephalopods contains a respiratory pigment - hemocyanin, which includes copper, so when oxidized, the blood turns blue.

Excretory system represented by two or four (in Nautilus) kidneys. With their inner ends they open into the pericardial sac (pericardium), and with their outer ends into the mantle cavity. Excretion products enter the kidneys from the branchial veins and from the extensive pericardial cavity. Additionally, the excretory function is performed by the pericardial glands formed by the wall of the pericardium.

Reproductive system, reproduction and development. Cephalopods are dioecious animals. In some species, sexual dimorphism is well expressed, for example in the Argonauta. The female Argonaut is larger than the male (Fig. 243) and during the breeding season, with the help of special glands on the tentacles, she secretes around her body a thin-walled parchment-like brood chamber for gestating eggs, similar to a spiral shell. The male argonaut is several times smaller than the female and has a special elongated sexual tentacle, which is filled with reproductive products during the breeding season.

Gonads and reproductive ducts are unpaired. The exception is the nautilus, which has preserved paired ducts extending from the unpaired gonad. In males, the vas deferens passes into the spermatophore sac, where spermatozoa are glued together into special packages - spermatophores. In cuttlefish, the spermatophore is checker-shaped; its cavity is filled with sperm, and the outlet is closed with a complex plug. During the breeding season, the male cuttlefish uses a genital tentacle with a spoon-shaped end to transfer the spermatophore into the mantle cavity of the female.

Rice. 243. Argonauta mollusk: A - female, B - male; 1 - funnel, 2 - eye, 3 - shell, 4 - hectocotylus, 5 - funnel, 6 - eye (according to Dogel)

Cephalopods usually lay eggs at the bottom. Some species exhibit care for their offspring. Thus, the female Argonaut bears eggs in the brood chamber, and octopuses guard the clutch of eggs, which are placed in shelters made of stones or in caves. Development is direct, without metamorphosis. The eggs hatch into small, fully formed cephalopods.

Modern cephalopods belong to two subclasses: the subclass Nautiloidea and the subclass Coleoidea. The extinct subclasses include: subclass Ammonoidea, subclass Bactritoidea and subclass Belemnoidea.

Subclass Nautilidae

Modern nautilids include one order Nautilida. It is represented by only one genus, Nautilus, which includes only a few species. The distribution range of Nautilus is limited to the tropical regions of the Indian and Pacific Oceans. There are more than 2,500 species of nautilid fossils. This is an ancient group of cephalopods, known since the Cambrian.

Nautilids have many primitive features: the presence of an external multi-chambered shell, an unfused funnel, numerous tentacles without suckers, and the manifestation of metamerism (four ctenidia, four kidneys, four atria). The similarity of nautilids with lower shelled mollusks is manifested in the structure of the nervous system from cords without separate ganglia, as well as in the structure of coelomoducts.

Nautilus is a benthopelagic cephalopod. It floats in the water column in a “reactive” way, pushing water out of the funnel. The multi-chamber shell ensures the buoyancy of its body and sinking to the bottom. The Nautilus has long been an object of fishing for its beautiful mother-of-pearl shell. Many exquisite pieces of jewelry are made from nautilus shells.

Subclass Coleoidea

Coleoidea means "hard" in Latin. These are hard-skinned mollusks without a shell. Coleoids are a thriving group of modern cephalopods, comprising four orders, which include about 650 species.

Common features of the subclass are: lack of a developed shell, fused funnel, tentacles with suction cups.

Unlike nautilids, they have only two ctenidia, two kidneys and two atria. Coleoidea have a highly developed nervous system and sensory organs. The following three orders are characterized by the largest number of species.

Order Cuttlefish (Sepiida). The most characteristic representatives of the order are cuttlefish (Sepia) and Spirula (Spirula) with rudiments of an internal shell. They have 10 tentacles, two of which are hunting tentacles. These are nektobenthic animals, stay near the bottom and are able to actively swim.

Order Squids (Teuthida). This includes many commercial squids: Todarodes, Loligo, etc. Squids sometimes retain a rudiment

shells in the form of a horny plate under the skin on the back. They have 10 tentacles, like the previous squad. These are mainly nektonic animals that actively swim in the water column and have a torpedo-shaped body (Fig. 244).

Order Octopoda (Octopoda). They are an evolutionarily advanced group of cephalopods without traces of a shell. They have eight tentacles. Sexual dimorphism is pronounced. Males develop a sexual tentacle - a hectocotylus. This includes a variety of octopuses (Fig. 245). Most octopuses lead a bottom-dwelling lifestyle. But among them there are nektonic and even planktonic forms. The order Octopoda includes the genus Argonauta - the argonaut, in which the female secretes a special brood chamber.

Rice. 244. Squid Loligo (from Dogel)

Rice. 245. Octopus (male) Ocythoe (according to Pelzner): 1 - tentacles, 2 - funnel, 3 - hectocotylus, 4 - sac, 5 - terminal filament

Practical significance of cephalopods

Cephalopods are game animals. The meat of cuttlefish, squid and octopus is used as food. The global catch of cephalopods currently reaches more than 1,600 thousand tons. in year. Cuttlefish and some octopuses are also harvested for the purpose of obtaining ink liquid, from which natural ink and ink of the highest quality are made.

Paleontology and phylogeny of cephalopods

The most ancient group of cephalopods is considered to be nautilids, whose fossil shells are already known from Cambrian deposits. Primitive nautilids had a low conical shell with only a few chambers and a wide siphon. Cephalopods are thought to have evolved from ancient crawling testate mollusks with simple conical shells and flat soles, like some fossil monoplacophorans. Apparently, a significant aromorphosis in the emergence of cephalopods was the appearance of the first partitions and chambers in the shell, which marked the beginning of the development of their hydrostatic apparatus and determined the ability to float up, breaking away from the bottom. Apparently, the formation of the funnel and tentacles occurred in parallel. The shells of ancient nautilids were varied in shape: long conical and flat, spirally twisted with a different number of chambers. Among them there were also giants up to 4-5 m (Endoceras), which led a benthic lifestyle. Nautilids underwent several periods of prosperity and decline in the process of historical development and have existed to this day, although they are now represented by only one genus, Nautilus.

In the Devonian, in parallel with the nautilids, a special group of cephalopods began to be found - bactrites (Bactritoidea), smaller in size and less specialized than the nautilids. It is assumed that this group of cephalopods descended from common as yet unknown ancestors with nautilids. Bactrites turned out to be an evolutionarily promising group. They gave rise to two branches of cephalopod development: ammonites and belemnites.

The subclass of ammonites (Ammonoidea) appeared in the Devonian and died out at the end of the Cretaceous. During their heyday, ammonites successfully competed with nautilids, whose numbers were noticeably declining at that time. It is difficult for us to judge the advantages of the internal organization of ammonites only from fossil shells. But the ammonite shell was more perfect,

Rice. 246. Fossil cephalopods: A - ammonite, B - belemnite

than that of nautilids: lighter and stronger. The partitions between the chambers of ammonites were not smooth, but wavy, and the lines of the partitions on the shell were zigzag, which increased the strength of the shell. Ammonite shells were spirally twisted. More often, the spiral whorls of ammonite shells were located in one plane, and less often they had the shape of a turbo-spiral (Fig. 246, A). Based on some body imprints of the fossil remains of ammonites, it can be assumed that they had up to 10 tentacles, possibly two ctenidia, beak-shaped jaws, and an ink sac. This indicates that ammonites apparently underwent oligomerization of metameric organs. According to paleontology, ammonites were more ecologically diverse than nautilids, and included nektonic, benthic and planktonic forms. Most ammonites were small in size, but there were also giants with a shell diameter of up to 2 m. Ammonites were among the most numerous marine animals in the Mesozoic, and their fossil shells serve as guiding forms in geology for determining the age of strata.

Another branch of cephalopod evolution, hypothetically derived from bactrites, was represented by the subclass of belemnites (Belemnoidea). Belemnites appeared in the Triassic, flourished in the Cretaceous, and died out at the beginning of the Cenozoic era. In their appearance they are already closer to the modern subclass Coleoidea. In body shape they resemble modern squids (Fig. 246, B). However, belemnites differed significantly from them in the presence of a heavy shell, which was overgrown with a mantle. The belemnites shell was conical, multi-chambered, covered with skin. In geological deposits, remains of shells and especially their terminal finger-like rostrums, which are figuratively called “devil’s fingers,” have been preserved. Belemnites were often very large: their length reached several meters. The extinction of ammonites and belemnites was probably due to increased competition with bony fish. And in the Cenozoic, a new group of cephalopods entered the arena of life - coleoids (subclass Coleoidea), devoid of shells, with fast reactive movement, with a complexly developed nervous system and sensory organs. They became the “primates” of the sea and could compete on equal terms as predators with fish. This group of cephalopods appeared

in the Cretaceous, but reached its peak in the Cenozoic era. There is reason to believe that Coleoidea have common origins with belemnites.

Environmental radiation of cephalopods. The ecological radiation of cephalopods is presented in Figure 247. From primitive shelled benthopelagic forms capable of floating due to the hydrostatic apparatus, several paths of ecological specialization have emerged. The most ancient ecological directions were associated with the radiation of nautilids and ammonites, which swam at different depths and formed specialized shell forms of benthopelagic cephalopods. From benthopelagic forms there is a transition to bentonectonic ones (such as belemnites). Their shell becomes internal, and its function as a swimming apparatus weakens. In return, they develop a main mover - a funnel. Later they gave rise to shellless forms. The latter undergo rapid environmental radiation, forming nektobenthic, nektonic, benthic and planktonic forms.

The main representatives of nekton are squid, but there are also fast-swimming octopuses and cuttlefish with a narrow torpedo-shaped body. The composition of nektobenthos mainly includes cuttlefish, often swimming

Rice. 247. Ecological radiation of cephalopods

or lying on the bottom, to bentonecton - octopuses that crawl along the bottom more than swim. Plankton include umbrella-shaped, or gelatinous, octopuses and rod-shaped squids.