Molluscs 2BIbDF09 10/11

What are molluscs?

Molluscs are invertebrates animals that live in water, and mostly in marine environment. There is an estimated number of 100'000 species in the phylum Mollusca. Molluscs appeared in the earliest Cambrian period 545 million years ago. They have a bilateral symmetry and are soft-bodied animals, but most are protected by a hard shell. Some molluscs have external shells, like Nautiluses, Snails or Clams, but other have reduced or internal shells, like Cuttlefish or Squids. Others just lost it in mollusc evolution, like Octopuses or Slugs.

We can define four different groups of molluscs; the Gastropods, the Cephalopods, the Bivalves and the Polyplacophores. All of them have the same body plan separated in three parts. The first part is the foot which is the strong, muscular portion used for locomotion. The second one is the visceral mass, a soft-bodied portion that contains internal organs and the third one is the mantle, that covers and protect the visceral mass. Many molluscs feed by using a rasping organ called a radula. (cf appendix molluscs 1)

{ Annexe is FRENCH... You should say APPENDIX [[ Utilisateur:Pierre.brawand|Pierre.brawand]] 16 janvier 2011 à 21:09 (CET)

what is a shell? Is it an exoskeleton?

The Gastropods

The Gastropods form the group of molluscs the most important(over40,000 species).This group include the snails, whelks, limpets and the slugs. We call them Gastropods because it meaning stomach-footed. All the gastropods are oviparous animals.

We can find them nearly everywhere in the world because some of them live into the water and the others live in somewhere is humid. But their anatomy is a little bit different like their color, their way of the respiration.

Although most gastropods are herbivores or detritivores.They have a unique structure in their mouths called a radula.

The radula functions like a rasp to scrapee away algae plants cell or other type of food It is usually covered with teeth. Sea butter-flies are gastropodsd with areduced or absent shell but a large winglike foot that flaps and poweer swimming movements

How do they move? They move by muscle contractions, thanks to their ventrally flattened foot.That is why the land gastropods are very slow.

They can have a shell like the snails.

What are the differences between the Land gastropods and the sea gastropods?

The shell of the snails change. The Land snails have thinner shell than the marine snails. This shell is in the shape of spiral,which is used for protection.

{co|where did you get this information? In a French website? If this is the case, don't do that. I don't want you to translate. I want you to read in English and to synthesize the main information.}Pierre.brawand 16 janvier 2011 à 21:11 (CET)

The nudibranchs are marine gastropods and they have lost their shell but we called also sea slugs. snails: The snails are hermaphrodite. This means that every snail have a female part and a male part. The female part can produce eggs and the male part makes sperm. But it is impossible for only one snails reproduce itself, the snail has to meet an other.However during the mating the snails can change their sexual part thanks to that the two snails have their eggs fertilized, so they can leave ad lay some eggs.

The snail's body is kept moist by mucus, and that helps this animal to glide along as the foot ripples. The mucus which is produce by the snail helps its movement and reduces the risk of wound. When snails pull their bodies into their shells, they can close the operculum which is like a door. They have two pairs of tentacles of which the longest have eyes in the extremity.

Nudibranchs

The Cephalopods

Cephalopods differ from gastropods and bivalves. Unlike bivalves and most gastropods which have shells, most cephalopods, like the Octopus, miss it. Only a few cephalopods have a heavy external shell (like the Nautilus) or a small and internal shell (like the Squids). (search an appendix)

They are carnivorous predators made for speed and agility; cephalopods are know for their intelligence and ability to catch their fast-moving prey. They are found in all oceans of the world. All species of cephalopods have tentacles that they use to catch and hold prey; mostly fishes. The tentacles seem to be arms that help them to feed themselves and cling to surfaces, thanks to suction cups on the tentacles. Cephalopods are great predators thanks to their speed movement, camera eye-vision and their intelligence.

They move by jet propulsion, which is a complex muscular motion:

The seawater is first drawn into the mantle cavity over the gills. When the animal wants to move, it quickly contracts the mantle wall. The pressure of water within the mantle cavity is increased because there is only one orifice not sealed, the funnel or siphon, by which the water goes out, propeling the animal fast through the surrounding water. (appendix ?)

The cephalopods need to see quite well so that they can catch prey easily, thus they have what we call a "camera eye vision", which is a great sight they have thanks to their developed eyes, that don't differ so much with human eyes. Indeed, the eye of a cephalopod is composed of a two chamber system, separated by a lens, with a retina that catch light and a pupil with an iris. The structure of the retina show similarities too, but the human eye is more complex. In both, the pupil protects the retina from the amount of light.

Cephalopods are considered like the most intelligent invertebrates. They have a develop nervous system and an extremely complex brain, which is an advantage being predators. Thanks to their intelligence, they are able to adapt to their environment and they have the ability to "disappear", using camouflage as the main protection against predators.

The Bivalves

(Bivalves, including numerous species of clams, oysters, mussels and scallops, have shells divided into two halves hinged together. Most bivalves are sedentary, living in sand or mud in marine and freshwater environments. They use their muscular foot for digging and anchoring.)

make a link with the mussel observation made in class Pierre.brawand 30 janvier 2011 à 15:13 (CET)

References

• http://www.earthlife.net/inverts/mollusca.html
• Essential Biology with Physiology Campbell/Reece/Simon Second Edition
• http://www.dtmag.com/Stories/Marine%20Life/02-06-whats_that.htm
• http://www.waterencyclopedia.com/Ce-Cr/Cephalopods.html
• http://sharon-taxonomy2009-p3.wikispaces.com/Mollusca

good start but still a lot of work to do...! Pierre.brawand 16 janvier 2011 à 21:13 (CET)

still a lot to do...[ [Utilisateur:Pierre.brawand|Pierre.brawand]] 30 janvier 2011 à 15:13 (CET)

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