SECTION V 

 PHYLUM PLATYHELMINTHES 



A NUMBER of classes of Metazoa, some a little, others very de- 

 cidedly, higher in organisation than the Coelenterata, are commonly 

 regarded as constituting one great sub-kingdom or phylum the 

 Vcrmes or Worms. The groups ordinarily referred to the Vermes 

 differ, however, very widely from one another: points of agree- 

 i in 'lit, except such as are merely negative, are, in fact, frequently 

 hardly recognisable: and rather than group together under one 

 common designation such a heterogeneous assemblage of forms, it 

 sci -ins better to avoid the term Vcrmes altogether and to endeavour 

 to divide the "worms" into phyla the members of which shall 

 have points of positive resemblance to one another. The four 

 phyla Platyhelminthes, Nemathelminthes, Troclielminthcs, Mollns- 

 coida, and Annulata, with their appendices, all consist of forms 

 which are or have been comprised in the Vermes. They differ 

 from the Coelenterata in the presence of three well-developed 

 body-layers of which the middle one, or mesoderm, is of relatively 

 predominant importance: and, for the most part, in the much 

 higher stage of complexity attained by the various systems of 

 organs. The first four phyla present no metameric segmentation: 

 in^the Annulata, metamerism is more or less strongly pronounced. 



The Platyhelminthes or Flat-Worms are a group of soft-bodied, 

 bilateral, usually flattened animals, which are devoid of true 

 metameric segmentation (p. 41). With a sufficient degree of uni- 

 formity of structure to render the phylum a fairly compact and 

 well-defined one, there is yet a considerable range in complexity, 

 from the simplest forms, certain of which have been supposed to be 

 nearly connected with the Ctenophora among the Coelenterata, to 

 the highest, which have all the various systems of organs very 

 mueh more highly developed. The body is built up from 

 three embryonic lavers ectoderm, mesoderm, and i-ii<!n,l,Tm as in 



