CHAPTER 1 2 



ABERRANT ANIMALS— 

 THE MOLLUSKS AND ECHINODERMS 



All of the animals considered so far have PHYLUM MOLLUSCA 

 followed a series of rather logical steps, in 



which increasingly complex physiological The soft-bodied animals that compose 



needs have been satisfied by the develop- the phylum Mollusca include the snail and 



ment of new parts supplementing the basic clam, which are familiar to nearly every- 



plan of the lower phyla. Two very large one, as well as the lesser known squid, 



groups of animals, the mollusks and the chiton, and octopus. These animals are 



echinoderms, have solved these needs in a scattered through the oceans and fresh 



manner quite different from the other waters of the world, their large fleshy 



groups. Since they have not followed the bodies providing an abundant source of 



trend that has been obvious from amoeba food for man and other animals. There are 



through the arthropods, they are known as over 70,000 species of mollusks which vary 



aberrant animals. Both groups are biologi- rather widely in external appearance but 



cally successful: not only are there several have similar basic body plans. Perhaps the 



thousand species of mollusks and echino- most notable and striking thing about the 



derms, but they are spread over a great part mollusks is the lack of segmentation. Even 



of the earth. the chitons (Fig. 12-1), which appear ex- 



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