Mollusca

Anatomy

General characters

Although probably going down from the annélides, the majority of molluscs lost all traces of Métamérisation. They have a bilateral symmetry, but which can be faded by a torsion of the body.

Their tegument is soft. It contains many Glande S which secretes Mucus.

The molluscs are Cœlomate S, but them Cœlome is limited to a Péricarde, i.e. the Cœur is located in a cavity dug in fabric of origin Mésodermique. The general cavity of molluscs is more or less obliterated by conjunctive fabric, except for a part which wraps the heart (pericardium) and of another part, in relation to the two others, which constitutes the bodies excréteurs (néphridies).

General anatomy

Their body is subdivided in four parts:

the head which contains the sensors and the Bouche. It is absent at the Bivalves.
the Pied is a musculeux, typical body molluscs, intended for the Locomotion. It revêt of the very diverse forms according to the Espèce S.
the visceral mass, as its name indicates it, contains the Viscère S. It is contained in a thin tunic which one calls the coat. It is the coat which secretes the shell molluscs, which is used to them of protection and/or Squelette.

Entre the coat and the visceral mass, the Bourrelet palléal constitutes a Cavité palléale which protects the respiratory bodies, and where the Métanéphridie S (bodies excréteurs) emerge, the Intestin and genital conduits.

Cockle

Nervous system

The typical nervous system of a mollusc includes/understands cerebroid ganglia (which can amalgamate to form a brain) connected on the one hand to pedal ganglia, on the other hand with visceral ganglia, by a double collar périœsophagien.

Circulatory apparatus

Circulation is incomplete, lacunar. Heart start from short arteries but there are neither veins, nor capillaries.

Blood colorless, or is slightly coloured by Hémoglobine or dissolved Hémocyanine.

Reproductive cycle

The sexes are generally separate.

The eggs are more or less rich in vitellus, and the blossoming takes place after a more or less advanced stage of development.

When there is free larva (Trochophore, Véligère), this one resembles the trocophore of the annélides much.

Evolution

The molluscs would go down from an organization of the " type; ver". It is thought that they go down from the Annélide S from the traces of Métamérie discovered at the Monoplacophores. One estimates their appearance at least 500 My starting from a common ancestor (adaptive radiation).

The functionality which seems to have conditioned primitive molluscs appears to be the Radula: a body functioning like a grater, left language carrying chitinous teeth, which makes it possible the animal more effectively to be nourished. Compared to the " vermiformes" primitive, which can only gober one food fragmentary, the radula gives an adaptive advantage, insofar as it makes it possible to tear off food on coherent preys (sponges, algae,…). The molluscs thus invented the art of brouter.

The other functionality characteristic of molluscs is the shielding, making it possible to protect itself from predatory credits: the acquisition of plates limestones protecting the back. These primitive molluscs were to thus resemble Polyplacophore S (a kind of snail which can roll itself in ball like a hedgehog or a woodlouse), but this type is now very marginal.

While adapting to various forms of life, they gradually conquered all the types of medium: especially present in seamen circle, the Gastropodous and the Bivalves then succeeded in adapting to fresh water. In their adaptive radiation, the molluscs gave rise to the following important classes:

gastropodous (snails, slugs, patellas…) continue to crawl, and are characterized by a more advanced cephalization. The only innovation that their the evolution brought is that this snaking is done on a specialized body, the foot. The plates limestones of the primitive carapace were simplified with the wire of time, which led to these generally spiral shells. Some Gastropods conquered the terrestrial environments towards the end of the Crétacé. It could be besides that they are the first animals with being left water. Their adaptation to the terrestrial environment is not yet completely succeeded.

bivalvular (moulds, oysters…) became sedentary, and misent on protection that brings to them the calcareous shell, at the point to move practically more. Their lifestyle approaches that of the anemones, even of sponges, consisting in filtering ambient water. In this evolution, they lost their head, become useless, and the eyes are more present only in degenerated form, in some species. The Bivalve S constitute an interesting case where a functional regression (loss of displacement specific to the vermiform structures) results in an evolutionary success.

the cephalopods (octopuses, squids, cuttlefish…) learned how to swim, and are the predatory ones. Capacity to catch preys which can seek to escape met a strong constraint evolutionary on what characterizes this group: good eyes, and a powerful brain able to coordinate the movements of hunting. The common shell of the invertebrates, which one finds at the Argonaute, tends to be profiled at a peak, to be reduced as at the cuttlefish, to even disappear completely as at the Poulpe.

It will be noticed that the bivalves and the cephalopods lost to them Radula, character which had been the cause of the initial radiative explosion of molluscs.

Classification

The Phylum of Molluscs was created by Georges Cuvier (1769-1832) in 1795.

There are currently 8 Mollusc classes:

the Solénogastres (350 known species living in all the seas)
the Caudofovéates (100 known species living in all the seas of the sphere)
the Polyplacophore S (900 known species living between 0 and - 3.000 m)
the Monoplacophore S (15 known species living in the oceanic pits)
Gastropodous S (103 000 known species having a world distribution)
the Cephalopod S (786 known species, all navy living in all the seas except the Black Sea)
Bivalvular S (12 000 species living out of fresh water and in all the seas of the world)
the Scaphopode S (400 very marine species)

Solénogastres and Caudofovéates were in the past gathered in the same class: the Aplacophores.
On the contrary, the Eumollusques gather all molluscs except for Solénogastres and of Caudofovéates.

The Conchifères are a Sous-embranchement gathering all Eumollusques except Polyplacophores.
The Amphineures are the second subphylum of Molluscs and gather Aplacophores and Polyplacophores.

The bivalves and Scaphopodes can be gathered under the term Diasomes.

Vernacular name

See also: List of the vernacular mollusc names

As for fish, the vernacular names are enough can homogeneous and depend much on the areas where they are used. They can be unknown from one area to another or not indicate the same species.

Place des molluscs in the animal world

See too

The science which treats molluscs names the Malacologie and that which treats only Coquillage S, the Conchyliologie.

Taxonomic references

External bonds

'''MalaCo :''' the electronic newspaper of French continental malacology
Voir a detailed diagram of the anatomy interns of a mollusc

Simple: Mollusc Zh-min-nan: Nńg-tea tōng-bu̍t

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