GENERAL ZOOLOGY 



Mouth 



ABORAL 



Mouth 



Mouth 



*Anus 

 ABORAL 



Anus 



ORAL 



Fig. 16.1 1. Cleaned tests of some echinoids which approach bilateral symmetry in some external 

 features. A and B, a sand dollar, Clypeaster, in which both mouth and anus lie on the 

 "median plane" of the oral surface. C, a heart urchin, Spatangus, with the mouth displaced 

 "anteriorly" and the anus at the opposite end of the body. D and E, a sand dollar, 

 Echinarachmus, with the mouth centrally located and the anus at the "posterior" edge of the 

 flattened body. These modifications may be interpreted in relation to the habits of the 

 animals, which either inhabit burrows or live just under the surface of sandy bottoms. 



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