The family of shellfish, or as they are also called - chitons, has about 500 species. According to experts, this figure is small. Especially when compared to other families. Chitons or shellfish are inhabitants of the tidal zone in the seas and oceans. The presence of strong armor in these animals is due to the aggressive environment in which they exist. The constant blows of the surf can only withstand creatures with reliable protection. In this article, we will look at the description of shellfish, photos in their natural environment and features of the species.
Clam lifestyle
Mollusks have underdeveloped sense organs. This is due to the fact that they have a passive lifestyle. They practically do not move. In addition, representatives of shellfish do not have balance organs. Their organs of vision are quite complicated. The eyes representa biconvex lens with a vitreous body surrounded by pigment cells. It is noteworthy that in shell mollusks, shell plates continue to grow not only at the stage of formation, but throughout life, and new so-called shell eyes regularly appear on their edges. By the end of its life, the chiton may have more than eleven thousand eyes. Science still does not know their purpose. The olfactory organs possessed by the shellfish are very sensitive to water quality. They are located in the back of the body of this marine life. The organs of taste are in the mouth.
Shellfish prefer to live exclusively in aquatic environments with a high s alt content. In addition, the temperature of the water is also important. It should not drop to 1 degree. They mainly live in the tidal zone in places of surf. This is due to the fact that in this way the mollusks receive the necessary amount of oxygen, and gas exchange in regularly stirred water is much better. However, some classes of shellfish have adapted to life at depth. But there are very few such species. Chitons that live in intertidal zones are large, have a strong, well-developed shell and muscles. They are provided with all means of protection against sea waves.
Habitats
Representatives of the shellfish family can be found in a wide variety of soil types. However, they prefer rocks and pebbles with a flat surface, on which it is easier for them to gain a foothold. The coloration of mollusks has a masking effectagainst the background of coastal pebbles. This saves them during low tides from their main enemy - birds. Due to their 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 shellfish
The body of most types of chitons is almond-shaped. 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, diamond-shaped plates are formed, folding into a bizarre mosaic. They have sharp edges, with which shellfish are attached to the substrate.
The head is disc-shaped and is located at the end of the abdominal part. It is noteworthy that the shell mollusk does not have head eyes. The head is separated from the leg, which occupies the main part of the surface of the abdomen, with a transverse suture. The function of the leg is not to move, but to stick the chiton to rocks and pebbles. Between the leg, the girdle and the head section there is a mantle groove, at the bottom of which the gills are located. There can be a very different number of them 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 nerve cords,departing from him. They are located on the sides of the body under its very surface and are connected to each other in its back part. In addition, shellfish have pedal trunks that are found in the leg muscles. The cerebral cords are connected to them and the pleural cords, forming a nerve ring. It is noteworthy that chitons also have ganglia. They are located on the pharynx and send nerve impulses to the radula and pharynx.
The device of the 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 one ventricle. The atria are located absolutely symmetrically on the sides and are connected by atrioventricular openings to the ventricle. The aorta passes from it and enters the atrium through one of the vessels that bring oxidized blood from the gills. The peripheral system of shellfish is underdeveloped and almost completely replaced by lacunae.
Features of the respiratory system
Papace molluscs have a large number of gills, which are located on both sides of the body in the mantle furrow. It is noteworthy that only a pair of gills, which are located in the back, are 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 is a clam shell made of
The sink, consisting of 8 plates, has a multilayer structure. Inner layers on98% are calcium carbonate. They also contain conchiolin, but only in the form of a layer between the layers. The topmost of them is the thinnest, consists of 100% conchiolin. This provides it with elasticity and protection against alkalis and acids found in the aquatic environment.
The plates that make up the shell have many grooves into which the protrusions of the skin of the mollusk go. They are called aesthetes. In some species of these animals, the layer of plates, which is located at the bottom of the shell, protrudes beyond the upper layers, forming pterygoid outgrowths. They serve to fasten the muscles. In many types of mollusks, shell reduction occurs during life. During this process, the plates change their shape, decrease, and their surface is completely overgrown with a mantle.
Reproduction
The bulk of the types of shellfish are dioecious creatures. At the same time, their fertilization is carried out externally, without mating as such. Many chitons lay their eggs directly into the water, where they swim freely. It is noteworthy that there are species of molluscs that have 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 that lay them in water. As a rule, their number in the first does not exceed two hundred. Species that lay directly in water can produce up to 1,500 eggs.
The development of a mollusk is expressed in transformation. First, a larva appears from the egg, outwardly similar to worms. In the abdominal part, it has a protrusion with cilia. This is the beginning of the future leg. On her back, several depressions are formed, which gradually increase the plates for the shell. At this stage, the chiton has a disc-shaped shape, but when it passes to the next, its shape becomes like an amygdala. The front is more rounded. There is a head. The narrower back is covered with a shell, the leg is more and more clearly visible below.
Chitons are one of the oldest animals on our planet. Scientists have proven that the first shellfish are found in the Paleozoic era, and this is about 400 million years ago.
