CHAPTER 20 



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Phylum Mollusca. 



Snails, Squids, 



Octopuses, 



and Others 



HE soft-bodied animals, which comprise 

 the phylum Mollusca, include the snails, 

 slugs, clams, mussels, oysters, octopuses, and 

 squids (Fig. 170). Most of them are bilater- 

 ally symmetrical; and, with the exception of 

 one class, are unsegmented. Many possess 

 shells of calcium carbonate. At first sight, 

 mussels, clams, snails, and squids do not 

 appear to have much in common, but closer 

 examination reveals several structures which 

 are possessed by all. One of these is an organ 

 called the foot, which in the snail is usually 

 used for creeping over surfaces; in the clam, 

 generally, for plowing through the mud; and 

 in the squid for seizing prey. In each there 

 is a space called the mantle cavity between 

 the main body and an enclosing envelope, 

 the mantle. The anus opens into the mantle 

 cavity. 



Mollusks are among the most abundant of 

 all animals and may be found on land, in 

 fresh water, and in the sea. Many of the 

 90,000 species of this phylum are of eco- 

 nomic importance. They serve as food for 

 man— oysters, clams, and scallops; they pro- 

 vide material for making pearl buttons; and 

 they produce pearls. Some species are in- 

 jurious. Although the various types of mol- 

 lusks differ widely in appearance and struc- 

 ture, they can be reduced to a single plan 

 (Fig. 171). The varied activities of the dif- 

 ferent types indicate how well they are 

 adapted to their environments both mor- 

 phologically and physiologically. The larvae 

 of all mollusks, except the cephalopods, are 

 particularly interesting because they pass 

 through trochophorc and veliger stages, 

 which suggests that they are related to the 

 annelids and certain other phyla. 



The mollusks are divided into 6 classes, 

 according to symmetr}' and the character of 

 the foot, shell, mantle, gills, muscles, radula, 

 and nervous system. The mollusks follow 

 the arthropods in this discussion, not be- 

 cause they are more complex or because they 

 belong in this position, but for convenience, 

 lliey could just as well come before the 

 arthropods. 



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