Molluscs

Author: Sarah Moran
__**EXAMPLES**__: Molluscs are made up of eight living classes, the largest and most important of which are gastropods, cephalopods, and bivavles.

The Blue Ringed Octopus, a deadly poisonous species of Cephalopod, a class of Mollusc

A common snail, of the class Gastropod

__**HABITAT**__: Molluscs are found in a wide variety of habitats, from marine and freshwater to terrestrial places. By and large, the vast majority are found in water. Even in this respect, there is variety. Some are found in shallows while others make their lives in the ocean's depths.

__**TYPE OF COELOM:**__ Molluscs are coelomates and thus have true coeloms - they have fluid filled cavities within the mesoderm that contain the internal organs and act as a hydrostatic skeleton. They are also protostomes, meaning their first embryonic opening or blastopore becomes the mouth.

Most molluscs have a sort of exoskeleton in the form of a shell which rises from the mantle, a covering that encloses the visceral mass of the body. However, some, mostly the cephalopods, have only an endoskeleton made of cartilaginous structures that lend form and support.
 * __ENDOSKELETON or EXOSKELETON__:**

Again, molluscs vary widely in complexity of nervous systems. The bivavles have relatively limited nervous systems, with only three sets of ganglia, or nerve chords, that wind from the "head" of the body to the appendages. Most have some sort of basic eyes that sense light. Gastropods have up to six pairs of ganglia with heavy concentrations of nerve chords in the anterior of their bodies. Cephalopods are the most complex - some octopi even have the mental capacities of house cats. Cephalopods have highly developed brains made mostly of bundles of ganglia. They have very sophisticated eyes, comparable to that of humans, that can be huge in size.
 * __TYPE OF NERVOUS SYSTEM__:**

__**FEEDER**__: Almost all types of feeding mechanisms are found among molluscs. The lesser forms, like the bivavles, are herbivorous filter feeders. This means that cilia direct microscopic food particles into their mouths and then down into the digestive system. Some are herbivores, feeding on plants; others are carnivores, eating and attacking others animals as prey. Cephalopods in particular are excellent predators. They use their tentacles to grasp prey and tear them apart with their beak-like jaws. The herbivores most often have a radula, a toothed tongue that is used to scrape algae off of rocks and into the digestive tract. Finally, some are parasitic, feeding off of other animals.

__**SYMMETRY**__: All molluscs have bilateral symmetry - they are symmetrical if cut right down the center of the body. Either side mirrors the other.

__**REPRODUCTION**__: Various methods of reproduction are found in molluscs. Some, like snails, are hermaphrodites. This means individuals contain both female and male sex organs. Fertilization thus occurs internally within the animal. In bivalves, some species have "brood pouches" - a small pouch between the gills - in which the young develop. Others practice external fertilization with separate sexes. Cephalopods are always either one sex or the other and often have complex mating rituals. Fertilization occurs when the male uses one of his arms to transfer a packet of sperm called the spermatophore to the female's mantle cavity. Embryos then develop within gelatinous egg mass of the female.

__**CIRCULATORY SYSTEM**__: Most molluscs have an open circulatory system - they have a hemocoel that bathes the organs in blood to oxygenate them. Oxygen enters the blood through the gills and is pumped through the heart, then goes to the hemocoel. However, cephalopods, with their highly active lives, have a closed circulatory system. The blood is always enclosed with blood vessels or a heart. Squids actually have three hearts that pump blood to various parts of the body.

__**ENDOTHERMIC or EXOTHERMIC**__. Molluscs are unable to regulate their own internal body temperature and must rely on the external environment to provide them heat. Thus, they are exothermic.

__**SEGMENTATION**__: Molluscs are unsegmented animals. They are, however, made up of three main body parts - a head-foot, the visceral mass, and the mantle. Additionally, some (like cephalopods) have tentacles and other appendages.

__**WORKS CITED**__: //Blue Ring Octopus//. Photograph. //It's Nature//. Web. 27 Mar. 2011. . Photograph. //Littlewood's Nature Guide//. 26 Apr. 2008. Web. 27 Mar. 2011. . Mader, Sylvia S. "30.2 Molluscs." //Biology//. 9th ed. New York: McGraw-Hill, 2007. 540-43. Print. "Phylum Mollusca." //Infusion//. Web. 27 Mar. 2011. .