CHAPTER VIII 

 MOLLUSCA 



The mollusca are soft bodied (mollis, soft) animals, usually with 

 an external shell or an internal "pen" or bone, found both on land and 

 in fresh and salt water. They are primitively bilaterally symmetrical, 

 but this symmetry is often obscured in the adults. They are unseg- 

 mented. Although the various classes seem at first glance, very 

 dissimilar, yet they have certain structures in common and they have 

 a similar mode of development. Two characteristic organs of the 



PIG. 28. Fresh-water mussel, Anodon. XM- / foot, protruded; g, line 

 of growth, only a few of which have been drawn; ds, dorsal siphon; Ig, hinge liga- 

 ment; u, umbo. 



phylum are the foot, variously modified for different uses, and the 

 mantle, inclosing between it and the main body the mantle cavity 

 into which the digestive and urogenital organs open. The characters, 

 mainly of the foot, mantle, shell, gills and nervous system, determine 

 the classification of the phylum, which is one of great economic 

 importance. 



Five classes are usually described. 



i. Lamellibranchiata or Pelecypoda, Fig. 28. The bivalves, 

 such as oysters, clams, mussels, etc. Usually bilaterally symmetrical. 



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