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 The Phylum Mollusca

Etymology:- From the Latin Molluscus meaning soft of body.|
Characteristics of Mollusca:- 
1)Bilaterally symmetrical. 
2)Body has more than two cell layers, tissues and organs. 
3)Body without cavity. 
4)Body possesses a through gut with mouth and anus. 
5)Body monomeric and highly variable in form, may possess a dorsal or lateral shells of protein and calcareous spicules.  6)Has a nervous system with a circum-oesophagal ring, ganglia and paired nerve chords.  7)Has an open circulatory system with a heart and an aorta.  8)Has gaseous exchange organs called ctenidial gills. 

9)Has a pair of kidneys. 
10)Reproduction normally sexual and gonochoristic. 
11)Feed a wide range of material. 
12)Live in most environments|

The mollusks constitute one of the largest phyla of animals, both in numbers of living species (at least 47,000, and perhaps many more) and in numbers of individuals.      A significant characteristic of mollusks is their possession of a coelom, a fluid-filled cavity that develops within the mesoderm.  The coelom not only functions as a hydrostatic skeleton but also provides space within which the internal organs can be suspended by the mesenteries.         All mollusks have a soft body (their name is derived from the Latin word mollus, meaning "soft"), which is generally protected by a hard, calcium-  containing shell.  In some forms however, the shell has been lost in the course of evolution, as in slugs and octopuses, or greatly reduced in size and internalized, as in squids.     Structurally, mollusks are quite distinct from all other animals.  However, all modern mollusks have the same fundamental body plan.  There are three distinct body zones: a head-foot, which contains both the sensory and motor organs; a visceral mass, which contains the well-developed organs of digestion, excretion, and reproduction; and a mantle, a specialized tissue formed from folds of the dorsal body wall, that hangs over and enfolds the visceral mass and that secretes the shell.  The mantle cavity, a space between the mantle and the visceral mass, houses the gills; the digestive, excretory, and reproductive systems discharge into it.    

    Mollusks are also characterized by a toothed tongue, the radula, composed primarily of chitin.  The radula serves both to scrape off algae and other food materials and also to convey them backward to the digestive tract.  In some species, it is also used in combat. Digestion and Excretion 

    The digestive tract is complete and ciliated, with a mouth, anus and complex stomach. The pattern of the stomach varies according to the mollusks diet.  Food is taken up by cells lining the digestive glands arising from the stomach, and then is passed into the blood.  Undigested materials are compressed and packaged, then discharged through the anus into the mantle cavity and are carried away from the animals in the water currents.  This packaging of wastes in solid form prevents fouling of the water passing over the gills.          Excretory functions are carried out by a pair of nephridia, tubular structures that collect fluids from the coelom and exchange salts and other substances with body tissues as the fluid passes along the tubules for excretion. The nephridia empty into the mantle cavity.

    = Food
    = Radula
    = Odontophore "belt"
    = Muscles

Nervous System and Sensory Capability 
    Mollusks have a relatively complex nervous system, which varies from species to species reaching the height of complexity at the octopus. The octopus is thought to be among the most intelligent of all invertebrates, with a mental capacity likened to that of a domestic cat.  Sensory ability in some mollusks (notably the cephalopods) is considerable, with a variety of organ systems, as well as large, complex eyes. The eyes of the giant squid are the largest in the animal kingdom, approaching the size of...
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