534 



THE QUARTERLY REVIEW OF BIOLOGY 



doubt of the fundamental similarity of 

 type. The head end, or scolex, of the 

 tapeworms is always radially symmetrical, 

 but the body, composed of a chain of 

 proglottids, is flattened and bilaterally 

 symmetrical, although the difference be- 



are formed within the original unit 

 instead of in a linear series. The flukes 

 and their allies always retain obvious 

 traces of radial symmetry, especially in 

 the digestive and in the nervous systems. 

 Here we come upon a most important 



__Turt>ellarian8 

 „„.--" ***"' Nematodes *»CT"— — 



.Vir Cestodes £•---—*""" 

 Fig. i. Relations of the Various Types of Animal Structure 



twecn the so-called dorsal and ventral 

 surfaces is but little marked. 



(x) The parasitic flatworms known as 

 flukes have a peculiar development which 

 is essentially similar to the strobilization 

 of the jelly-fishes, except that the buds 



fact. In the radially symmetrical animals 

 in which the polyps are of different types, 

 each fitted to perform a different kind of 

 work, these polyps take three forms. 

 They are (a) nutritive, or sack-like, 

 specialized for feeding, (£) reproductive, 



