COMPARATIVE ANATOMY 



CHAPTER XVI 

 THE PLATYHELMIA— DISTOMUM HEPATICUM 



It has been shown that in the Coelenterata, of which Hydra 

 and Obelia were chosen as examples, there are only two 

 primary layers, ectoderm and endoderm ; that the chief axis 

 of the body is represented by a Hne drawn through the mouth 

 to the opposite end of the body^ the organs being disposed 

 radially with regard to that axis; and that there is only one 

 cavity in the body, the gastrovascular cavity, which subserves 

 the functions of digestion and circulation. 



In the phylum Platyhelmia we have to deal with animals 

 in which a third layer, the mesoderm or mesoblast, is 

 interposed between ectoderm and endoderm, and the bulk 

 of the organs, including the muscular and reproductive 

 systems, are wholly derived from this middle layer. Further 

 than this, the body is generally elongated to a greater or 

 less extent and flattened from above downwards. There 

 is thus an upper or dorsal surface, a lower or ventral surface, 

 and right and left sides. Most Platyhelmia move in a definite 

 direction, and thus an anterior or head end, and a posterior 

 or tail end, may be distinguished. The principal axis of 

 the body is antero-posterior, and the chief organs of the body 

 are disposed more or less symmetrically in pairs on either 

 side of this axis, so that the animals may be described as 

 bilaterally symmetrical. That is to say, if any one of 

 them is divided into two halves by an antero-posterior cut, 

 the right half will correspond exactly with the left half. 



Although in the Platyhelmia there is no obvious cavity 

 or coelom, in which the gut and other organs lie, the cavity 

 of the gut serves only for digestive functions, and cannot 

 be regarded as a gastrovascular cavity like that of the 

 Coelenterata. On the other hand, there is no specialised 

 circulatory or blood-vascular system, and in default of this 



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