SECTION V 



PHYLUM PLATYHELMINTHES 



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

 decidedly, higher in organisation than the Co3lenterata, 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 

 agreement, except such as are merely negative, are, in fact, fre- 

 quently 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 five phyla Platyhelminthes, Nemathel- 

 minthes, Trochelminthes, Molluscoida, and Annulata, with their 

 appendices, all consist of forms which are or have been comprised 

 in the Vermes. They differ from the Ccelenterata 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 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 

 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 

 a peculiar kind the water-vascular or protonephridial system is 



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