The gastropods, or snails, are considered a rather large class of animals, which today has approximately 100 thousand species. Almost all the ancient representatives of the group were marine organisms, which in the process of evolution switched to terrestrial and freshwater existence. Parasitic mollusks are also known, but there are not so many of them. Interestingly, some species switched to the aquatic mode of life for the second time, their ancestors were terrestrial organisms.
Gastropods: body structure
The body of representatives of this type is divided into the head section, trunk, and muscular leg. Most species have one or two pairs of tentacles on their heads. As for the body, it is asymmetric and is completely located in the shell. The shell of the gastropod is integral and forms a spiral with a different number of curls. The leg serves to move and fix - on the lower side there is a special sole. In case of danger, the head and leg are pulled into the sink.
The mantle cavity of the animal is located in the lower turns of the shell. The ducts of the excretory system, as well as the anus, are opened here. Some members of the class also have ducts of the reproductive system. Interestingly, part of the mantle in marine inhabitants turned into gills, while in terrestrial inhabitants it contains lungs.
The integument is represented by the epithelium, which is rich in specific glands producing mucus.
Gastropods mollusks: internal structure
The digestive system begins with the oral cavity, the opening of which is located on the underside of the head section. In the throat there is a specific formation called radula, which is a chitinous grater with which food is ground and passes into the esophagus. Next comes the stomach (the liver ducts open here) and the intestines, which ends with an opening.
The excretory system in most species is represented by one kidney, although there are animals with two organs. The circulatory system is of the lacunar type and consists of the heart, the system of lacunae and blood vessels.
The respiratory system is represented by gills or lungs. In marine representatives, as a rule, you can notice ctenidia, or primary gills. In the same species, which again switched to aquatic existence, the mantle performs the function of gills. Ground representatives are characterized by a more complex respiratory system, which consists of a lung and a tracheal tube system, which diverges and carries air to all parts of the body.
The nervous system, which is classified as a diffuse-nodal type, is also well formed. Most animals have five pairs of primary ganglia. It is worth noting well-developed sensory organs, including the eyes and tactile receptors.
Gastropod mollusks can be either dioecious (marine species) or hermaphrodites (terrestrial and freshwater representatives). Fertilization, as a rule, occurs in the body of the female, after which she lays eggs. Interestingly, at one time the gastropod mollusk lays several eggs at once, forming a clutch.
The eggs are covered with a capsule with a small cap at the top. Sometimes, unfertilized, empty capsules are attached to the outer sides of the masonry using mucus. This technique is protective in nature - a predator gnaws a few empty eggs, and then leaves. As for development, it can be both direct and with the formation of larvae.
Gastropods: the most popular species
The most famous representative of the class is the so-called grape snail. It is worth noting that large groups of these animals can harm the grape harvest. At the same time, specially grown snails in some countries are considered an expensive delicacy.
And some housewives are familiar with slug - a small mollusk with a reduced shell that feeds on plants. This is a real pest of vegetables, therefore, chemicals are used to destroy populations.