CHAPTER Y 



PHYLUM PLATYHELMINTHES 



THE name Platyhelminthes (Gr. -rrAarvs, flat; eX/uvs, 2X/uv#os, a 

 worm) means simply "flat worms." The word worm is a popular 

 expression not capable of any very exact definition. It connotes in 

 the popular, mind a low wriggling animal without conspicuous ap- 

 pendages. The animals belonging to the phylum of Platyhelminthes 

 are almost always of a flattened shape. They agree with the 

 Coelenterata in possessing only one opening to the alimentary 

 canal and this, as in Coelenterata, functions as a mouth through 

 which nourishment is taken in. Between ectoderm and endoderm 

 there intervenes a mass of tissue which strongly recalls the tissue 

 making up the body of a Ctenophore. This tissue is called paren- 

 chyma and its basis is a mass of stellate cells embedded in a ground 

 substance. The processes of many of these cells are metamorphosed 

 into muscle fibres, and these muscle fibres are arranged in longi- 

 tudinal, circular and vertical layers. The Platyhelminthes are 

 distinguished from Ctenophora by three main features. (1) They 

 have a definite nervous system separated from the ectoderm, which 

 consists of two large closely connected masses in the anterior part of 

 the body called the brain, from which two bands of nerve fibres run 

 backwards along the two sides of the body which are termed lateral 

 nerve cords. The swellings termed ganglia (Gr. ya'yyXioi/, a knot) 

 are produced by an accumulation of the bodies of nerve cells. 

 The cords consist mainly of their outgrowths which constitute nerve 

 fibres, although cell bodies are not entirely restricted to the ganglia 

 but are also scattered along the course of the cords in lesser 

 numbers. (2) They possess reproductive organs which do not 

 discharge directly either to the exterior or into the alimentary canal, 

 but which are connected with the exterior by long complicated 

 tubes termed genital ducts. (3) They possess a definite excretory 

 system. With regard to the last-named point it is to be noted that 



