From the Latin molluscus ("soft"), a mollusk is a metazoan with soft tegumentates, which can appear naked or covered by a shell. It presents bilateral symmetry and, as an adult, a non-segmented body.
Mollusks are coelomed protostome invertebrates and form one of the broadest phyla in the animal kingdom. They are the most numerous invertebrates behind arthropods, including species such as octopuses, slugs, squid, mussels and clams. In total, scientists estimate that there are about 100,000 living species.
Thanks to their physiological characteristics, mollusks can live in a wide variety of environments. In this way, individuals of this group can be found at more than 3,000 meters above sea level and also in ocean depths of more than 5,000 meters.
Some of the peculiarities of mollusks that differentiate them from other animals are the presence of a feeding organ known as the radula, which is formed by curved chitinous teeth; the development of a muscular foot; and the existence of a calcareous shell secreted by the mantle (although, in certain species, it may be absent).
The muscular foot demonstrates the great evolutionary capacity of mollusks. In principle, it was crawling, similar to that of gastropods. Eventually, she could diversify into a digger standing (in bivalves) a swimmer standing (in pelagic gastropods) or even a foot split in tentacles (in cephalopods).
It should be noted that, in terms of reproduction, mollusks can be unisexual (bivalves) or hermaphrodites (snails), with the capacity, even, of self-fertilization in some cases. Some are provided with separate sexes and fertilization takes place externally, in the water; there are some species in which fertilization occurs internally; and in the case of hermaphrodites, they are endowed with both sexes and can carry out fertilization without the need for another individual, which is the case with snails.
First mollusks and evolution
The first fossils of mollusks that have been found come from the Cambrian period, that is to say around 600 million years ago. In fact, one of the largest organisms found in this period was the anomalocaris, a mollusk whose name means "strange shrimp" and was about 60 cm long, had a blanket-shaped body, a pair of large eyes arms terminated in clamp.
If we try to roughly describe the similarities shared by most mollusks, we can say that they have a head with a mouth (in some cases equipped with a tongue and minimal teeth with which it can gnaw) and brain ganglia; a membrane that receives the name of mantle and that covers and protects its internal organs (visceral mass); they have a single foot that allows them to move through water or land, depending on their habitat.
According to their different habitats, the type of food also varies. Land mollusks are herbivorous, among them we can find species of snails and slugs of different types. Seafarers can present extremely varied types of nutrition. Clams and mussels, for example, feed on filtering plankton, marine snails feed on algae, and squid and other mollusks are predators, that is, carnivores.