SECTION V 



PHYLUM PLATYHELMINTHES 



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

 cidedly, higher in organisation than the Coelenterata, were formerly 

 regarded as constituting one great sub-kingdom or phylum — the 

 Vermes or Worms. The groups ordinarily referred to the Vermes 

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

 ment, 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 

 is usually considered to be more expedient to avoid the term 

 Vermes altogether, and to endeavour to divide the " Worms " into 

 phyla the members of which shall have points of positive resem- 

 blance to one another. The four phyla Platyhelminthes, Nemathel- 

 minthes, Trochelminthes, Molluseoida, 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 meta- 

 meric segmentation (p. 43) : 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. 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 complexit}^ 

 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 

 much more highly developed. The body is built up from 

 three embryonic layers — ectoderm, mesoderm, and endoderm — as in 

 all higher groups of animals. An excretory vascular system of 



