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Soft-Bodied Animals — The Mollusks 



The phylum Mollusca is very extensive, containing nearly 90,000 spe- 

 cies, a greater number than is found in any other phylum except the 

 Arthropoda. The name is derived from a Latin word mollis, which means 

 soft and is appropriately chosen in view of the soft body parts of these ani- 

 mals. Most of them have a shell on the outside of the body to protect these 

 delicate organs. Because of the presence of this shell many of the marine 

 forms are called shellfish. The phylum includes such widely divergent 

 forms as the snail, octopus, squid, oyster, and clam. These may not seem 

 to have many characteristics in common that would cause them to be 

 grouped together in the same phylum, but a survey shows them to be 

 quite similar in fundamental structure. They all have a mantle, which 

 surrounds a part of the body called the visceral hump, that contains im- 

 portant internal organs. All the species that are able to move about 

 have a foot as a locomotion organ that extends down ventrally from the 

 visceral hump. The mantle secretes a liquid, which hardens to form 

 a shell that surrounds the mantle in most species ; but there are some, like 

 the squid, that do not have an external shell, although in these forms the 

 mantle is somewhat tough and offers a degree of protection. The soft 

 bodies of many of them make delicious eating, and the mollusks (com- 

 mon name) furnish us a very important source of food. The Mollusca 

 include a rather highly specialized group of animals that have adopted 

 a pattern of life quite different from other phyla ; thus their organ 

 systems show specialization that sets this phylum apart as a side branch 

 of the animal tree. 



Class — Pelecypoda 



This name means "hatchet foot" and refers to the shape of the foot in 

 these mollusks, but the common name "bivalve" is a little more descrip- 

 tive because the shell is composed of two halves or valves hinged to- 

 gether at the dorsal surface. Fresh water as well as marine forms are 

 included in this class. They are of great economic value, for not only 

 are most of them edible, but also valuable pearls and mother-of-pearl 

 ornaments are obtained from them. 



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