Gastropods

The gastropods are the largest (about 80%) and most diverse class of the phylum Mollusca. This class comprises the snails, which have a singe coiled shell into which the animal can generally withdraw, and the slugs, whose shells have been reduced to an internal fragment or completely lost in the course of evolution. Gastropods are not only diverse with respect to the number of species alive today, they are diverse in terms of their size, shape, color, body structure and shell morphology.

The majority of the gastropods are asymmetric and dextraly coiled, and they occupy all marine habitats from equator to the poles, from shall to deeper waters. Multiple groups have colonized freshwater habitats, while others have evolved the ability to breathe air and have colonized terrestrial habitats, including mountains and deserts. Most marine gastropods breathe via their gills, however freshwater and terrestrial species are an exception to this rule and breathe using a rudimentary lung. Those gastropods that breathe using a lung are called pulmonates. Gastropods are among the few groups of animals to have become successful in all three major habitats: the ocean, fresh waters, and land.

This mollusks range in size from 0,33 mm to 1m length, most are mobile, but some cement them selfs to hard substrates while others are external or internal parasites. Gastropods may be herbivores, carnivores, parasites, filter feeders, detritivores or chemoautotrophs (rare). Those that feed on plants and algae use their radula to scrape and shred their food. Gastropods that are predators use a siphon to suction food into the mantle cavity and filter it over its gills. Some predatory gastropods feed on shelled prey by boring a hole through the shell to locate the soft body parts inside.

The coil of the shell usually twists in a clockwise direction, so that when viewed with the apex (top) of the shell pointing upward, the opening of the shell is located on the right. Many gastropods (such as sea snails, terrestrial snails, and freshwater snails) have a hardened structure on the surface of their foot called an operculum that work's as a “trap-door”. The operculum serves as a lid that protects the gastropod when it retracts its body within its shell. The operculum seals the shell opening to prevent desiccation or deter predators. The operculum may be solid and heavy (eg. Turbinidae) or pliant and horny (eg. Muricidae or Trochidae). Sometimes it is very small and obsolete and it isn't big enough to seal the aperture, likes happens in some cones.

      

Chicoreus saulii (Muricidae)

Conus textile (Conidae)

During the larval stage, all gastropods undergo torsion, a very rapid process that results in the rotation of the visceral mass and mantle on the foot. The result is that the mantle cavity (including anus) lies in the anterior body, over the head and mouth, and the gut and nervous system are twisted. . This twisting means that the head is between 90 and 180 degrees offset relative to their foot. Torsion results in asymmetrical growth, with more growth occurring on the left side of the body. Torsion causes the loss of the right side of any paired appendages. Thus, although gastropods are still considered to be bilaterally symmetrical (that's how they start out), by the time they become adults, gastropods that have undergone torsion have lost some elements of their "symmetry". The adult gastropod ends up configured in such a way that its body and internal organs are twisted and the mantle and mantle cavity are above its head. It should be noted that torsion involves the twisting of the gastropod's body, it has nothing to do with the coiling of the shell Some species reverse torsion ("detorsion"), but evidence of having passed through a twisted phase can be seen in the anatomy of these forms. Torsion in gastropods has the unfortunate result that wastes are expelled from the gut and nephridia near the gills. A variety of morphological and physiological adaptations have arisen to separate water used for respiration from water bearing waste products.

Gastropods are dioecious (sexually distinct), and some forms are hermaphroditic. Hermaphroditic forms exchange bundles of sperm to avoid self-fertilization; copulation may be complex and in some species ends with each individual sending a sperm-containing dart into the tissues of the other.

Gastropods are divided into the following basic groups:

• Patellogastropoda
• Vetigastropoda
• Cocculiniformia
• Neritimorpha
• Caenogastropoda
• Heterobranchia

Turbo marmoratus ( Vetigastropoda)

Mitra mitra (Caenogastropoda)