124 THE INVERTEBRATA 



Crustacea, a coelomoduct is supplemented or in great part replaced 

 by an ectodermal component, but there is no evidence that this 

 component represents a nephridium. 



The body constituted by the foregoing elements has usually a 

 bilateral symmetry^ though this is rarely exhibited completely by 

 all the systems. In the Coelenterata and Echinodermata, however, 

 there is a radial symmetry. It is interesting to find that a sessile 

 life, for which such symmetry seems particularly advantageous, is 

 characteristic of the Coelenterata, and was probably adopted by the 

 ancestors of all the Echinodermata. The terms ventral and dorsal^ 

 which belong by right respectively to those aspects of a bilateral 

 animal which are normally turned to and from the ground or sub- 

 stratum, are sometimes conveniently applied to a pair of structures 

 by which two sides may be distinguished in the body of an animal 

 whose symmetry is predominantly radial. They should, however, 

 never be applied to the oral and aboral aspects of such an animal. 



Meristic repetition of organs of the body is common in metazoa. It 

 may, as in parts of the body of annelids, affect practically all systems, 

 so that there is a complete segmentaiion of the body into similar 

 somites^ or may be confined to certain organs. In the latter case it is 

 important to distinguish between {a) the repetition of single organs 

 in an unsegmented animal, as the ctenidia and shell plates are in- 

 dependently repeated in the mollusc Chiton, and {b) the condition, 

 presented for instance by the Vertebrata and by much of the body 

 of many arthropods, in which a formerly more complete segmentation 

 now affects only some of the systems to which it at one time extended. 

 The student should beware of thinking that the segmentation of all 

 animals which present the phenomenon is derived from that of a 

 common ancestor. The strobilization of the Cestoda in preparation 

 for the detachment of reproductive units is a very different matter 

 from the segmentation of the Annelida, and that again is far from 

 being, as is sometimes assumed, certainly the same thing as the seg- 

 mentation of the Vertebrata. 



The anterior end of a bilateral animal is the site of the principal 

 sense organs, of the " brain", and usually also of the mouth, and is 

 often obviously differentiated as a head. In a segmented animal 

 this cephalization may extend to one or more of the anterior somites ; 

 and these usually become part of the head, losing their individuality 

 in the way mentioned in the preceding paragraph, and only 

 betraying their existence by the presence of certain of their organs 

 (ganglia, appendages, etc.). 



