The family of shell-shaped mollusks, or, as they are also called, chitons, has about 500 species. According to experts, this figure is small. Especially when compared with other families. Chitons or shell mollusks are inhabitants of the tidal zone in the seas and oceans. The presence of strong armor in these animals is due to the hostile environment in which they exist. Constant blows of the surf can withstand only creatures that have reliable protection. In this article we will consider the description of shell-mollusks, photos in the natural environment and species features.
Shellfish lifestyle
Mollusks have underdeveloped sensory organs. This is due to the fact that they have a dominant passive lifestyle. They practically do not move. In addition, representatives of shell-mollusks do not have balance organs. It is quite difficult for them to have organs of vision. The eyes are a biconvex lens with a vitreous body surrounded by pigment cells. It is noteworthy that shell-shell mollusks continue to grow not only at the formation stage, but throughout life, and new so-called shell eyes regularly appear on their edges. By the end of his life, a tunic can have more than eleven thousand eyes. Science still does not know their purpose. The olfactory organs possessed by the shell mollusk are very sensitive to water quality. They are located at the back of the body of this marine creature. The organs of taste are in the mouth.
Shell mollusks prefer to live exclusively in an aquatic environment with a high salt content. In addition, the temperature of the water is important. It should not fall to 1 degree. They mainly live in the tidal zone in the surf. This is due to the fact that in this way the mollusks receive the necessary amount of oxygen, and gas exchange in regularly mixed water is much better. However, some classes of shellfish have adapted to life in depth. But such species are very few. Chitons living in intertidal zones are large, with a strong well-developed shell and muscles. They are provided with all means of protection from sea waves.
Habitat
Representatives of the shell-mollusk family can be found in a wide variety of soil types. However, rocks and pebbles with a flat surface on which they find it easier to fix are preferred. The color of the mollusks has a masking effect on the background of coastal pebbles. This saves them at low tide from their main enemy - the birds. Due to its color, the ability to stick firmly to the surface of rocks and a strong shell, these animals rarely become victims of predators. However, there are cases when mollusk shells were found in the stomachs of starfish and some fish species.
The structure of the shell mollusk
The body of most species of tunics has an almond-shaped shape. The main part is hidden under the surface of the sink. It consists of eight plates stacked on top of each other like tiles. Only the marginal zone of the mantle, or, as it is also called, the belt, remains unprotected. On its lower part rhomboid plates are formed, folding into a bizarre mosaic. They have sharp edges with which shell-shaped mollusks are attached to the substrate.
The head is disc-shaped and located at the end of the abdominal part. It is noteworthy that the shell-eye does not have head eyes. The head is divided with the leg, which occupies the main part of the surface of the abdomen, with a transverse seam. The function of the leg is not to move, but to suck the tunic to the rocks and pebbles. Between the leg, the belt and the head section, there is a mantle groove, at the bottom of which gills are located. There may be a very different amount depending on the type of mollusk.
Nervous system
The nervous system consists of a cerebral cord, which is located in front of the pharynx and pleural neural cords extending from it. They are located on the sides of the body under its very surface and are connected, with each other in its rear part. In addition, shell mollusks have pedal trunks that are located in the muscles of the leg. Cerebral cords connect with them and pleural, forming a nerve ring. It is noteworthy that ganglia exist in tunics. They are located on the pharynx and send nerve impulses to the radula and pharynx.
Circulatory system
The heart is located in the pericardium on the back of the body, in the back. It is expressed by two atria and 1 ventricle. The atria are located absolutely symmetrically on the sides and are connected by the atrioventricular openings to the ventricle. From it passes the aorta and enters the atrium along one of the vessels that bring oxidized blood from the gills. The peripheral system of shell-mollusks is underdeveloped and is almost completely replaced by gaps.
Features of the respiratory system
Shell mollusks have a large number of gills, which are located on both sides of the body in a mantle groove. It is noteworthy that only a pair of gills, which are located in the rear, is homologous. In turn, the remaining pairs are secondary and develop from the skin when there is a need to enhance gas exchange. According to biologists, almost every species of the shellfish family has a different number of these gills.
What does a mollusk shell consist of?
The sink, consisting of 8 plates, has a multilayer structure. The inner layers are 98% calcium carbonate. They contain conhiolin, but only in the form of a layer between the layers. The uppermost of them is the thinnest, consists of 100% conhiolin. This provides it with elasticity and protection against alkalis and acids in the aquatic environment.
The plates of which the shell consists have many grooves into which the protrusions of the mollusk skin cover. They are called aesthetics. In some species of these animals, the plate layer, which is located at the bottom of the shell, protrudes beyond the upper layers, forming pterygoid growths. They serve to fasten muscles. In many types of mollusks, shell reduction occurs during life. During this process, the plates change their shape, shrink, and their surface is completely overgrown with a mantle.
Breeding
The bulk of the types of shell mollusks are dioecious creatures. At the same time, they are fertilized externally, without mating as such. Many tunics lay eggs directly in the water, where they swim freely. It is noteworthy that there are species of mollusks that place eggs in the mantle cavity, and already the larva sets off for free swimming. An interesting fact is that, according to biologists, those mollusks that carefully store eggs in the mantle cavity have significantly fewer eggs than those who lay in water. As a rule, their number in the first does not exceed two hundred. Species that lay directly in the water can produce up to one and a half thousand eggs.
The development of the mollusk is expressed in transformation. First, a larva emerges from the egg, similar in appearance to worms. In the abdominal part, it has a protrusion with cilia. This is the germ of the future leg. On her back, several indentations form, which gradually build up plates for the sink. At this stage, the tunic has a disc-shaped shape, but when it moves on to the next, its shape becomes similar to the amygdala. The front part is more rounded. There is the head. The narrower back is covered with a sink, the foot is more and more clearly shown below.
Chitons are one of the oldest animals of our planet. Scientists have proven that the first shell-mollusks found in the Paleozoic era, and this is about 400 million years ago.