Shell shape and type are some of the criteria used in the classification of mollusks.
These animals are currently divided into three classes: gastropods, the shellfish and the cephalopods.
The unique spiral shell is a typical feature of the gastropod group. For this reason, they are called univalves (uni means "single", and valve, "piece").
Gastropods include the snail and the snail; The slug, although not shelled or very small, is also included in this group.
Gastropods are wetland aquatic or terrestrial animals. Aquatic ones breathe through gills, while terrestrials breathe lungs.
The head of the slug, snail and snail has two pairs of tentacles, similar in appearance to antennae. The eyes are at the ends of the longer pair of tentacles.
Snail and its tentacles
In the mouth there is the radula, a type of "scraper tongue" that facilitates the feeding of these animals.
Electron microscopy of the scraper radius.
Bivalves have a two-piece shell enclosed by strong muscles (bi means "two", and valve, "piece"). They are aquatic beings and generally live in the marine environment.
They are animals filters, that is, they remove the food from the water. They have no head or radula (they are the only mollusks devoid of this kind of tongue). Its visceral mass is fully protected by the shell. The foot expands outward as the shells open.
The breathing of these animals is gill; The shells allow a stream of water to circulate between the gills, which absorb and filter the dissolved oxygen in the water. In relation to reproduction in general, the sexes are separated, and the union of gametes, that is, fertilization, is external.
In water, the female releases the eggs, and the male releases the sperm. The cells meet, fertilization occurs and the eggs form.
Cephalopod is a word of Greek origin; comes from Kephale, which means "head", and from can, "foot". Designates a group of mollusks of which octopus and squid are part.
The shell may not exist (as in the octopus), be internal and small (as in the squid) or external (as in the nautilus).
Cephalopods have large heads, well-developed eyes and radula inside the mouth. They have eight, ten or more tentacles, which are elongated "arms".
These animals have the closed circulation - that is, blood only circulates inside vessels, unlike other mollusks.
The breath is gill. They have a quite developed nervous system compared to other invertebrates. In addition to sight, the smell is well refined.
These mollusks usually have separate sexes and fertilization is internal. Research indicates that some species of octopus take care of their young, protecting them from predators.
As a defense resource, some mollusks rely on camouflage. When changing color they are confused with the environment. Squid and octopus, for example, expel a dark substance into the water. This confuses the predators of these mollusks, allowing them to escape.