Moluscos. Con la casa a cuestas – Sala del Agua

ENGLISH:

Mollusks are soft-bodied animals ( divided in head, visceral mass and foot) with three unique characteristics in the animal kingdom by which they are identified: a muscular foot, a calcareous shell secreted by themselves and a peculiar feeding organ called radula.

Shape, size and colour of their shells are the result of the adaptation of the animals to their environment. This evolution has created weird and striking shapes that have fascinated the human beings since ancient times.

Mollusks are subdivided in eight classes.

• Gastropoda (snails) (75,000 species)
• Bivalvia (clams) (13,000 species)
• Scaphopoda (900 species)
• Cephalopoda (octopuses) ( 800 species)
• Polyplacophora (chitons) (600 species)
• Solenogastres (250 species)
• Caudofoveata (70 species)
• Monoplacophora (20 species)

Double Shell-Bivalves
All the bivalves are aquatic, and they are characterized by presenting two lateral valves, usually symmetrical, connected by a hinge and ligaments. Diversity of shapes and sizes is huge, and some of the species may reach 250 kilograms.

Gastropods
Gastropods or univalves, are the larger class of the mollusk’s phylum. There are more than 75,000 alive characterized species, but there may be up to 100,000, and 15,000 fossils. We can find them in all kind of environments ( even the deserts ), but mostly in salt and fresh water, but some of them have colonised the terrestrial environment, being the only group of mollusc with delegates in solid ground.

They present a cephalic area ( head), a  muscular ventral foot and a dorsal shell ( that can be reduced or even lost in the more evolved gastropods) . When the larvae suffer the torsion, that is the twist of the visceral mass over the foot  and the head. That allows them to hide before the head on the shell, giving them a clear evolutionary advantage.

Houses to all tastes
Although terrestrial gastropods are well-known , more than two thirds of its species lives in the water. Many sea gastropods are diggers and have siphons or tubes that extent further the cloak or even the shell, with the aim of obtaining  oxygen and food; siphons are used too to remotely detect prey. Diversity in s the shapes of the shells is due to the different evolutions and adaptations of the gastropods.

Not all carry their house 
Many gastropods have one piece-shell rolled in spiral , that usually open to the right ( when we observe the shell with the apex upwards). Many species have an operculum that act as a lid to close the shell; generally  is corneal material but in some species is calcareous. In some gastropods, like slugs or nudibranchs, the shell is reduced or completely atrophied and the body is elongated, making the torsion less evident.

They know how to protect themselves
The shell protects gastropods against predators and changes of the environment. Furthermore this shell may adopt different shapes with bumps and other structures that make them difficult to eat by predators.

Those without shell, like nudibranchs, have extravagant colours, either to disguise with the milieu ( cryptic)  or to warn about its dangerousness or toxicity ( aposematic).