Scientific mysticism. In Japanese cuisine there is a dish called “Dancing squid" The clam is placed in a bowl of rice and topped with soy sauce. The killed animal begins to move. Mystic? No. The sauce contains sodium.
The squid's nerve fibers react to it by contracting. Interaction is possible within several hours after the mollusk is caught from the sea. Have you ever caught pike?
When cutting it after 5-10 hours of lying out of the water, you find that the fish is twitching and its heart is beating. What about chickens running around after their heads are separated? So, there is no surprise in the posthumous dances of the squid. There is more of it in the creature's life. Let's talk about her.
Description and features of squid
It is called the primate of the sea. This indicates the upper stage of evolution that the squid occupies among cephalopods. In his class, the hero of the article has the most developed brain and even has a cartilaginous semblance of a skull.
Bone formation helps protect the thinking organ. It enables the squid's complex behavior. The animal is capable of cunning, deception and other intellectual tricks.
The trick is to combine the brain with other organs and functions of the animal. Yes, y giant squid the thinking center is shaped like a donut. The hole in the center is reserved for the esophagus. In other words, squid - shellfish, which eats through the brain.
The mouth of the hero of the article is so powerful that it resembles the beak of a bird. The density of the chitinous jaws allows them to pierce the skulls of large fish. The animal doesn’t care about thick fishing line either; it bites through it.
If the mollusk is nevertheless caught and ends up in a human mouth, embarrassment may occur. There have been several recorded cases of undercooked squid expelling sperm. Most precedents are recorded in Japan and Korea. Thus, in January 2013, mollusk sperm caused the hospitalization of a visitor to one of the restaurants in Seoul.
Sea squid in the “dancing” dish came to life when they began to chew it. The animal threw 12 spindle-shaped sacs of sperm into the mucous membrane of the restaurant visitor’s tongue and cheeks. The foreign substance caused a burning sensation. The woman spat out the dish and called the doctors.
No such cases have been recorded in Russia. There are regions where squid is a common dish, for example, the Far East. However, in our country, shellfish are cleaned of internal organs and boiled well. In Asian countries, squid are rarely cleaned.
The squid is classified as a cephalopod due to its body structure. The limbs do not come from him. The leg, transformed in the process of evolution into 10 tentacles, extends from the head of the animal, surrounding the mouth. The eyes of the mollusk have a familiar location. The structure of the organs of vision is similar to that of humans. At the same time, each eye is able to follow a different object.
The squid's body is a muscular mantle with a thin plate of chitin. It is located on the back and is the remnant of the shell. Squids do not need its frame, because they have developed jet propulsion.
By taking in water, contracting their bodies, and throwing out currents, mollusks swim faster than many fish. When were they created spaceships, the first rockets, scientists were inspired by squids. Next, details about their lifestyle.
Squid lifestyle and habitat
Lanterns could also be invented by looking at squids. Their bodies are equipped with photophores. In caught mollusks these are bluish dots on the skin. If big squid, photophores reach a diameter of 7.5 millimeters.
The structure of the “lamp” is reminiscent of the design of car headlights and lanterns. The light source is bacteria. They feed on squid ink. The mollusk fills the photophores with dark liquid when it wants to turn off the light. By the way, on the body of one mollusk there can be “lamps” of 10 different designs. There are, for example, “models” that can change the direction of the rays.
Some squids are even named after their ability to beam. Thus, the Firefly lives in Tayami Bay off the coast of Japan. More precisely, the mollusk lives at a depth of 400 meters. The colony washes ashore in June-July. This is the time for excursions, when tourists admire the bright blue waters of the bay. Scientists, at this time, are racking their brains as to why squids need photophores. There are several versions.
The most realistic: - light attracts cephalopod prey, that is, small fish. Second opinion: - the glow of squids scares away predators. The third assumption regarding the role of photophores is related to the communication of mollusks with each other.
400-500 meters is the standard depth limit at which it can live squid. Lives Below is just a gigantic view. Its representatives are also met 1000 meters under water. At the same time, the giant squid rises to the surface. Specimens measuring 13 meters long and weighing almost half a ton were caught here.
Most squid live at depths of about 100 meters, searching for muddy or sandy bottoms. Cephalopods flock to it in winter. In summer, squids rise to the surface.
Most of populations live in the North Atlantic Ocean. Here squid fishing carried out from to the North Sea. The Mediterranean is also rich in cephalopods.
Squids are also found in the Adriatic. It is difficult to track individuals because the animals migrate. The incentive to move is the search for food. In addition to fish, crustaceans, other mollusks, even relatives are used.
They are caught with two tentacles, injecting paralyzing poison into the victim. The squid tear off small pieces of flesh from the immobilized ones, slowly eating them. Having gained strength and waited until summer, squids begin to reproduce. Fertilization leads to the laying of eggs. It looks like a sausage, with a film on top and eggs inside. Afterwards, the parents leave.
After about a month, centimeter-long offspring are born, immediately beginning an independent life. It is possible only where the water salinity is 30-38 ppm per liter of water. This is why there are no squids in the Black Sea. The salinity of its waters does not exceed 22 ppm.
Types of squid
Let's start with the Pacific squid. This is what we usually see on the shelves of domestic stores. True, Russians are accustomed to calling the mollusk Far Eastern, after the place of catch.
The sizes of individuals start from a quarter and end with half a meter. This is along with the tentacles. Single squids reach 80 centimeters. The species lives at depths of up to 200 meters. The desired water temperature is 0.4-28 degrees Celsius.
The second of the main types of squid is Komandorskiy. It is also sold in Russia, sometimes outpacing the Pacific in terms of sales. The commander's species is smaller, growing to a maximum of 43 centimeters.
Standard size is 25-30 centimeters. Representatives of the species are distinguished by their ability to swim to depths of up to 1,200 meters. The young live near the surface. This is what mostly ends up on the shelves. The extermination of the species became the reason for the founding of the Komandorsky State Reserve. Squid fishing is prohibited there.
It remains to mention the European squid. Meat one individual weighs up to 1.5 kilos. The body length of the animal is 50 centimeters. The species swims to depths of up to 500 meters, usually staying at 100 meters. Individuals have short tentacles and a light body. In the Pacific species it is, for example, gray, and in the Komandorsky species it is reddish.
There are also Giant, Peruvian and Argentine squid. They can only be seen outside of Russia. The big view was talked about. Peruvian is inedible. Harm to squid lies in the ammonia taste and, in fact, the content of ammonia itself in the meat. The Argentinean variety is tender in taste, but loses it when frozen. Sometimes, Argentine clams are found in cans.
Squid feeding
In addition to fish, crayfish, worms and the like, the hero of the article catches plankton. Another dietary product is associated with benefits of squid For environment. Cephalopods feast on algae. Their squid is scraped off the rocks.
This improves the appearance of the bottom and prevents the water from blooming. If the target is a living creature, the hero of the article hunts from an ambush, tracking down the victim. The poison is injected by the radula. This is a set of cloves in an elastic shell. They not only deliver poison, but also hold prey while it tries to escape.
Reproduction and lifespan of squid
Squid seed sacs are located in a special tube. They could have met her while cleaning the carcasses. The length of the tube is from 1 centimeter to 1 meter, depending on the type of mollusk. Females receive seed material into a depression near the mouth, on the back of the head, or in the mouth.
The location of the fossa depends, again, on the species squid Price receiving sperm, sometimes months of gestation. Males do not select mates based on age. Often, the seed is transferred to an immature female and stored in her until reaching the reproductive period of life.
When children appear, the father may no longer be alive. Most squids die at the age of 1-3 years. Only giant individuals live longer. Their limit is 18 years. Old squids, as a rule, lose their taste and are somewhat harsh even with minimal heat treatment. So, they try to catch and prepare young animals for food. Its meat is considered dietary.
Squid calories is only 122 units per 100 grams of product. Proteins account for 22 grams. Fats make up less than 3 grams, and only 1 gram is allocated for carbohydrates. The rest of the mass is water. In the bodies of squids, like most animals, it is the basis.
Features of the structure of cephalopods
The head of most cephalopods is large, often somewhat wider than the mantle and separated from it by a cervical interception, but in octopuses, vampiromorphs, sepiolids and sepiodarids, it is fused with the latter. The head bears eyes, often very large, especially in deep-sea squids, and tentacles that surround the mouth of the mollusk like a crown. Most squid and all cuttlefish have 10 limbs, octopuses have 8, and nautilus have more than 90.
The inner surface of the arms of cephalopods (with the exception of Nautilus) is lined with suckers. They are located in 1-4 (rarely more) longitudinal rows. At the base of the arms the suckers are small, in the middle they are the largest, and at the ends they are tiny. Squids and cuttlefish, in addition to 8 arms, also have a pair of dexterous arms, or tentacles, which consist of a stem, smooth and, as a rule, devoid of suckers, and an expanded club, seated with suckers and hooks. In squids and cuttlefish, the suckers sit on stems and are equipped with chitinous rings with smooth or jagged edges. In some squids, the suckers turn into chitinous hooks, reminiscent of a cat's claws, which help them grab and hold large and slippery prey. Octopuses have stemless, “sessile” suckers, attached with a flat bottom to the surface of the arm and lacking chitinous rings and hooks.
A muscular conical tube is attached to the underside of the head, and sometimes grows, with its base extending into the mantle cavity. This funnel, or sifon, is the main mover of the cephalopod, its “jet engine”. The funnel of cephalopods, like the tentacles, is a homologue of the leg of mollusks. If the tube is a nozzle, then the mantle cavity is the “combustion chamber” of a living rocket. Sucking water into it through the mantle slit, the mollusk then forcefully pushes it out through the funnel. To prevent water from flowing back through the gap, the squid closes it tightly using special “buttons” located at the base of the funnel and on the inner surface of the mantle. The buttons have the appearance of tubercles and corresponding depressions and are called infundibular and mantle cartilages. When the mollusk contracts the muscles of the abdominal wall of the mantle, a strong stream of water shoots out from the funnel. The reactive force that arises in this case pushes the mollusk in the opposite direction. The funnel is directed towards the anterior end of the body, and therefore the mollusk usually swims with the rear end first. Jet shocks and suctions of water into the mantle cavity follow one after another with elusive speed, and the squid rushes like a rocket in the blue of the ocean. The musculature of the funnel is very perfect. With its help, the funnel can be turned in any direction, even backwards, which provides the animal with the ability to turn and reverse. The mouth opening of cephalopods is small.
The pharynx is muscular, equipped with two strong chitinous jaws, reminiscent of a parrot's beak and called the "beak". IN oral cavity on a special tongue-like protrusion - the odontophore - a radula is placed - a chitinous ribbon seated in rows of small denticles. Squids usually have 7 longitudinal rows of denticles (as an exception there are 5), Nautilus has 11 rows. With the help of the radula, food that enters the mollusk’s mouth and is moistened with saliva is transported further into the esophagus. Benthic octopuses use a very strong radula to drill into the shells of bivalves and gastropods and to scrape pieces of meat from under the shells of caught crabs. A thin tube stretches from the throat to the stomach - the esophagus, piercing the brain and liver on the way to the stomach. Therefore, cephalopods, despite their great appetite, cannot swallow their prey whole, but are forced to crush it into small pieces with their “beak” before putting it in their mouth. The eaten pieces of food then enter the muscular stomach, which receives digestive juices produced by the liver and pancreas. The activity of the enzymes of these glands is very high, and within 4 hours the food is digested. Absorption occurs in the blind process of the stomach - the caecum, as well as in the liver. Undigested food remains enter the intestine and are thrown out.
The liver is a large, oval, brownish organ usually located in front of the stomach. It performs several functions - it produces digestive enzymes, amino acids are absorbed in it, and it is also a repository of reserve nutrients. On the ventral side of the visceral mass lies an ink sac with a duct that drains into the intestine. Most cephalopods have an ink sac. It is absent only in Nautilus, Vampiroteuthis and some deep-sea octopuses. In the upper part of the mantle cavity there are gills - one on each side of the visceral mass (in Nautilus - two).
To the question about the complete structure of a squid asked by the author Kerosene stove the best answer is Squids (lat. Teuthida) are an order of decapod cephalopods. Typically measuring 0.25-0.5 m, giant squids of the genus Architeuthis can reach 20 meters (including tentacles) and are the largest invertebrates.
Squids have five pairs of tentacles. The fourth pair has lengthened in the process of evolution. The location of the suckers on the tentacles varies.
Squids have a streamlined torpedo-shaped body, which allows them to move at high speed with their “tail” forward, the main method of movement is reactive. A cartilaginous “arrow” runs along the squid’s body, supporting the body. It is called a gladius and is a vestige of an internal shell.
The coloration is varied; some deep-sea species of squid have a transparent body.
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The squid's body consists of a head, tentacles and torso. The body is a mantle sac, inside which are located the digestive organs, gonads, gills, gland that produces a peculiar coloring substance - sepia, and sepia itself in a special bag. The fins are located on the tail part of the mantle.
The squid's mantle is thick and muscular, especially on the ventral side. The relative thickness of the mantle wall is 4 - 5% of the mantle length.
The squid mantle consists of integumentary tissues and muscles, characterized by a complex structure. The skin is smooth and pigmented. The thickness of the skin, depending on the type of animal, reaches 2 - 17 mm.
The outer layer of the skin consists of a thin single layer columnar epithelium, covered with a thin layer of transparent mucus on top. Beneath the epithelium is the skin itself.
Squid skin consists of four layers. The two upper layers, between which the pigment grains are located, do not have an oriented fiber direction. The structure of the third layer of skin resembles muscle tissue with big amount cores. The fourth layer is characterized by a dense fibrous structure.
Between the skin and muscles there is a film connecting them.
The musculature, which makes up 98% of the thickness of the entire mantle, is formed by three types of muscle fibers intertwined with connective tissue.
The outer layer of muscle, located directly under the skin, is represented by a thin layer of longitudinal muscles. Next comes the musculature, consisting of alternating bands of circular muscles that are separated by radial muscle fibers. The annular muscles are the most developed in squid.
The inner surface of the mantle is covered with a thin shell connective tissue.
All types of muscle fibers are connected into a single whole, both among themselves and with the outer and inner sheaths of connective tissue fibers, arranged in the form of a three-dimensional lattice. Squid muscle fibers, averaging 3.6 µm in diameter, are approximately rectangular in shape and consist of rectangular, obliquely striated myofibrils arranged almost radially around a central cytoplasmic core. Myofibrils twist into a left-handed spiral lying at an angle of 16 - 17° to the main axis of the fibers.
The structure of the tissues of squid tentacles is even more complex compared to the mantle and determines their greater strength.
In squids, there are no species differences in the structure of muscle tissue.
The body of squids includes connective cartilaginous tissue that surrounds the brain, connects the mantle on one side to the head, and on the other to the funnel. There are also cartilages at the base of the fins.
According to the histological structure, the cartilage of cephalopods is close to the cartilage of vertebrates.
In the thickness of the mantle tissue on the dorsal side there is an internal shell - gladius (translated from Latin as “sword”). The gladius is feather-shaped and consists of a stem, a feather and sometimes a terminal cone.
The shape and size of the body, fins, thickness of the mantle sac, and shape of the gladius are the specific characteristics of squid.
As a rule, sexual characteristics are also expressed in the structure of the gladius - in females it is relatively wider than in males.
More about squid: http://www.teuthida.ru/index.php?name=Pages&op=page&pid=1
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 hindgut, 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 height to organization nervous system 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 this day 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: cuttlefish (Sepia officinalis), different kinds fast-swimming squids 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 - different types 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 in very various 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.