GENERAL ZOOLOGY 



typical representative such as the starfish, however, their morphological 

 peculiarities become intelligible. The indications are that the remote 

 ancestors of echinoderms were free-swimming, bilaterally symmetrical forms 

 with segmentally arranged coelomic pouches. At some time long antedating 

 the beginning of our fossil record, the ancestral forms apparently took up 

 an attached way of life, and the secondary radial symmetry developed 

 through some millions of years in connection with this fixed existence. The 

 anatomical peculiarities of modern echinoderms, even of those that are no 

 longer attached forms, are evidently related to these ancient adaptive changes. 

 Embryological and serological evidence indicates that echinoderms are more 

 closely related to the chordate evolutionary stem than to any other large 

 group of animals (see Fig. 7.3, p. 219). 



From an economic standpoint, echinoderms are unimportant, except for 

 starfishes, which sometimes destroy whole beds of shellfish. One of the few 

 echinoderms used as food bv man is a sea cucumber, gathered, dried, and 

 sold as heche-de-mer, or trepang. Neither do echinoderms serve as food for 

 many other animals; codfishes and related forms feed on starfishes, and the 

 dugong, an aquatic mammal, eats sea cucumbers. On the whole, however, 

 the spiny surfaces and the preponderance of skeletal material in the bodies 

 of echinoderms have helped them avoid destruction by predatory enemies. 



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