Characteristics of the type Mollusks

Type Mollusca, or Soft-bodied, is a large group of ancient inhabitants of our planet. They appeared about 450 - 500 million years ago. Fossil mollusks have been known since the Paleozoic era; in those days, brown, red and green algae dominated the Earth, and the first plants on land were just beginning to appear.

There are several points of view on the origin of mollusks. Some zoologists believe that the ancestors of mollusks were flatworms. Others suggest that mollusks evolved from annelids. Still others think that mollusks originate from ancestors common to annelids. These embryologists indicate the relationship of mollusks with annelids.

A typical mollusc larva (sailfish) is very similar to an annelid larva, bearing large lobes lined with cilia. The larva leads a planktonic lifestyle, then settles to the bottom and takes on the appearance of a typical gastropod.

Thus, mollusks are non-segmented bilaterally symmetrical soft-bodied animals (in gastropods the body is asymmetrical), having a shell, a mantle cavity, a reduced coelom, and an open circulatory system. Most likely they originated from common ancestors with annelids, which did not yet have the body divided into segments, the secondary body cavity was poorly developed and there were ciliated integuments.

Some characteristic features of the Mollusca type and the systematic position of the grape snail.

Now mollusks inhabit salt and fresh waters, land and master various types of nutrition (filter feeders, herbivores, carnivores). There are approximately 130 thousand species. The body length of mollusks ranges from 1 mm to 20 m.

The body of mollusks is unsegmented. Unlike its relatives - worms, movement is not carried out by the whole body, but only by a special part of it - the muscular leg. The remaining parts of the body, which themselves do not participate in the mechanism of movement, are either entirely or partially covered by a fold of skin - the mantle, which in the vast majority of forms secretes a calcareous shell; in this regard, the integument remains soft (soft-bodied type). Mollusks are characterized by a hard mineral shell that covers the animal’s body from the dorsal side. It plays the role of an exoskeleton. The shell, as a rule, consists of crystals of calcium carbonate. On top it is usually covered with a horn-like organic substance, and on the inside it is lined with a hard shiny calcareous layer - mother of pearl. The shell usually serves as a protective outer shell for the mollusk. It was this that allowed one order of gastropods to master the conditions of the terrestrial environment, protecting animals from drying out (land snails).

The phylum Mollusca is divided into three classes: Gastropods, Bivalves and Cephalopods, with the first two classes accounting for 98% of species.

The grape snail is a representative of the class Gastropods, subclass of pulmonate mollusks, order of stalked mollusks. Gastropods are the most numerous and diverse class of mollusks. It has about 90 thousand species. In the distant past, gastropods were inhabitants of the sea. Later, many of them adapted to life in fresh water bodies and on land. The sizes of different representatives of gastropods vary from 2-3 mm to several tens of centimeters. Thus, the object of our study, the grape snail, reaches a body length of 8-9 cm, and the size of its shell is up to 4 cm in width and 3.5 cm in length. The grape snail is part of the Helicidae family, which includes more than 300 genera of land mollusks. The Helix genus is one of the most widespread. Our species of grape snail is Helix pomatia L.

Habitat and lifestyle of the grape snail

Grape snails of the species Helix pomatia L. are inhabitants of the ground-air environment. Their habitats are in Australia and temperate countries throughout Europe (with the exception of northern regions), including southern Russia, Ukraine, Crimea and the Caucasus (from where the study subjects were brought). Snails live in gardens and vineyards, where they cause harm by eating, along with other greenery, young shoots of grapes and shoots of fruit trees and shrubs, but also on sea coasts and in high mountains, in shady parks and forests. On warm summer days, snails hide in damp and shady places, and at night they go in search of food. Grape snails from the southern regions of Russia in the wild consider primrose, zherikha and dandelion leaves to be their favorite plant foods. However, they are not as harmless as they seem, since they are also carnivores, proving themselves to be insidious predators. They eat their smaller counterparts (cannibalism), small insects and mammals.

When dry weather sets in, the snail pulls its body deep into the shell, secretes mucus that hardens in the air, closes the opening of the shell with it, and glues the edges of the shell to the surface on which it sits. When the animal is completely immobile, the life processes in its body slow down, and the tight blockage of the shell protects the body from drying out. Many snails are in a state of summer hibernation and are able to stay for several months until wet weather favorable for their life arrives.

With the onset of autumn cold weather, the grape snail prepares a winter shelter for itself by digging a hole in the ground, where it lies down to hibernate. She digs a hole with her foot, the sole of which is pressed tightly to the ground, and makes movements similar to crawling with force. If the soil turns out to be too hard, the snail flips over on its back and makes the same movements with its foot, raking fallen leaves on top of itself, which it uses as cover. Having burrowed, the snail is drawn into the shell and secretes a winter cap, epiphragm, containing lime, with its mantle edge. From inside, a second cap is then released from hardening mucus with a porous inclusion - a “window” located opposite the breathing hole. The snail falls into a stupor, but the metabolism in its body does not stop, although it occurs very slowly. The number of heart pulsations drops to one beat per minute. In the spring, before returning to activity, air is first drawn into the lung, and then the winter cap is shed.

Features of the morphology, anatomy and physiology of the grape snail.

The grape snail has three distinct body parts. These are the head, leg and bag-like torso.

The round, almost spherical calcareous shell of the grape snail reliably protects the soft body of the mollusk from enemies. The entire body of the snail, except for the head and legs, always remains inside the shell. The shell has an asymmetrical structure. The spiral whorls of the shell are convex, their outer part is smooth. The last whorl is large and swollen. The oldest part of the shell is its top; as the snail continues to grow, the edges of its shell grow; they become wider and constantly form new whorls. Depending on the shape of the shell, the parts of the body enclosed within it have an asymmetrical structure both in their general shape and in the location of the internal organs. The shell is painted in an orange-yellowish color, against which there are wide brown stripes along the whorls, the degree of expression of which, however, is very variable, so you can find specimens completely devoid of these stripes, i.e. with a single color.

A crawling grape snail clearly shows that its muscular organ of locomotion, the leg, occupies the entire ventral surface of the body. If a snail crawls on glass, you can see how waves of muscle contractions of individual parts of the leg run along the lower surface of its “sole” and how the snail moistens its path, secreting sticky mucus. The mucus-covered sole of the snail can be in close contact with the rough surface of the soil or bark and, acting like a suction cup, allows the snail to climb vertical tree trunks or stay on the underside of a leaf. In addition, the layer of mucus protects the thin skin of the sole from damage: the snail crawls, strictly speaking, not on the ground, not on the bark or on the surface of glass or sheet, but on a soft carpet of mucus secreted by itself.

At the front end of the body the snail has a separate head. There are two pairs of pincers and fingers on the head. The shorter tentacles of the first pair serve as an organ of smell, and at the ends of the long tentacles there are eyes. When a snail is worried, it retracts its tentacles inward, like the fingers of an empty glove pressed inward.

Digestive system. The mouth is placed on the underside of the head. You can see its work if you give a drop of sweet sugar solution towards a snail crawling on glass. Using a magnifying glass, we can examine the movement of the tongue, moving in different directions and armed with a special grater - radula (appendix). The grater consists of numerous small teeth, with their tips facing backwards, and acts like a rasp file used in wood processing . The snail's tongue scrapes the skin and pulp of leaves or other plant organs on which they feed; In addition, by pressing the tongue against the upper chitinous jaw, the snails can also tear off pieces of leaves. From the mouth, food passes into the pharynx, from where it then enters the stomach through the esophagus. The stomach passes into the intestines. The liver ducts flow into it. Liver secretions dissolve nutrients, which are then absorbed through the intestinal wall and into the blood. The intestine makes several loops and ends with the anus, which opens into the mantle cavity.

Respiratory system. The mantle cavity of the snail opens on the right side of the body. Grape snails breathe atmospheric air, and therefore they do not have gills in the mantle cavity, and respiratory gas exchange takes place in the mantle itself. Its free edge fuses with the body wall, leaving a small opening leading into the mantle cavity. Numerous blood vessels develop in the mantle, and the mantle cavity becomes the pulmonary cavity. This is how the lung is formed. Gas exchange occurs in the lung - saturating the blood with oxygen and releasing it from carbon dioxide. This type of breathing, when gas exchange occurs through the walls of the respiratory cavity into which air enters, is called pulmonary breathing. Thus, the mantle cavity performs the function of the lung.

The circulatory system is not closed: blood flows not only through the vessels, but also in the cavities between organs (lacunae), where gas exchange and transfer of nutrients to the cells of the body occur. The heart of the snail consists of an atrium and a ventricle. The aorta departs from the heart, it branches into arteries, after which the blood enters small cavities located among the connective tissue. There the blood gives off oxygen and is saturated with carbon dioxide. Next, the blood flows through the venous vessels to the lung, where the blood is enriched with oxygen and gets rid of carbon dioxide. Then the blood flows through the veins to the heart. The heart rate of the snail is 20-40 times per minute.

Excretory system. Due to the asymmetry of the body, only the left kidney is retained in the grape snail. At one end, this kidney, through a wide ciliated funnel, communicates with the pericardial sac (the remainder of the body cavity), where metabolic products are collected; at the other, it opens into the mantle cavity on the side of the anus. The pericardium is a remnant of the coelom. Therefore, we can say that the excretory systems of mollusks and annelids are similar in structure.

Nervous system and sensory organs. The nervous system of the grape snail is of a scattered-nodular type. It consists of several large nerve ganglia (gang l i e v), interconnected by nerve bridges, and numerous nerves. Due to the twisting of the body, the nerve bridges between some nodes form a cross. There are organs of balance and vision, receptors for chemical sense and touch.

Reproduction

Fertilization in grape snails is internal. These animals are hermaphrodites. The single reproductive hermaphroditic gland produces both sperm and eggs. Fertilization in these mollusks is cross-fertilization: each of the mating individuals plays the role of both male and female, so the genetic material of different individuals is exchanged. Regarding the reproduction of grape snails, numerous observations have been made of their behavior during the mating period, when they begin the so-called “love game” characteristic of many land snails. The snail's desire to mate is immediately evident in its behavior. She crawls slowly, as if in search of something, often stops halfway and waits for a long time in one place, slightly raising the front part of her body. If two such snails accidentally meet, they immediately begin the “love game” that precedes the act of fertilization. Both of them stretch upward, one against the other, and take a characteristic position, touching parts of the sole and feeling each other with tentacles and oral lobes. However, these movements stop after a short time, the animals fall and, pressing their soles tightly against each other, remain motionless for about a quarter to half an hour. After this period of rest, the previous game is resumed again, and this whole process lasts about two hours, when, finally, the one of the animals that has reached greater excitement does not stick a “love arrow” into the body of its partner, which also increases his excitement. After a short pause, the act of copulation itself occurs, with each snail playing the role of both male and female. Copulation itself in a grape snail lasts only a few minutes. After this, however, a long time passes until spermatophores are completely exchanged in completely exhausted animals. Only after this do they spread out in different directions, but at the same time, especially strong waves continue along the leg for a long time - contractions, thanks to which, probably, spermatophores containing sperm penetrate more easily into the seminal receptacle. Fertilized eggs, containing a supply of nutritional material for the future embryo and each individually covered with a protective shell, are laid in the ground in a hole specially dug by the snail, the hole of which is closed when the laying is completed.

In favorable conditions, snails can mate several times a month.

A clutch of several dozen white eggs, several millimeters in diameter, enclosed in a hard shell, the snails place on moist soil, in a pre-dug hole.

After 3-4 weeks, depending on the soil temperature, the eggs will hatch into tiny snails, miniature versions of the adults. They quickly spread out and begin to lead an independent life.