PLATYHELMINTHES 209 



appears to take place through the mucous membrane of 

 the mouth. 



To the Platyhelminthes belong also the Tapeworms or 

 Cestoda, of which Tcenia solium, found in man, 



Cestoda. 1 t 1 1 1 1 ■ 



is an example. In the adult state this worm may 

 reach a length of many feet. It lives in the intestine, to 

 whose wall it is attached by a head or scolex, provided with 

 four suckers and a crown of hooks. Behind the head is a 

 narrow neck, followed by a long chain of joints ox proglot- 

 tides which it buds off. The younger of these, near the 

 head, are small, but they grow larger as they are pushed 

 farther from the head by the formation of new joints. The 

 body is covered with a cuticle, under which lies a layer of 



«y«. 



p/i.K. 



Fig. 128. — A turbellarian (Planaria polychroa) swimming. — From 

 Shipley and MacBride. 



c. si., Ciliated slit at side of head ; eye ; g.o. , genital opening ; »z., mouth, at end of 

 outstretched pharynx ; ph.s., sheath into which pharynx can be withdrawn. 



very deep cells with longitudinal muscle fibres between 

 them and a circular layer of muscle below them. Inside 

 this is a mass of parenchyma like that of the fluke, in which 

 are embedded the excretory, generative, and nervous systems. 

 There is no alimentary canal, nutriment being absorbed 

 through the surface of the body. The excretory system is 

 of the same type as that of the fluke, with flame cells and a 

 larger and a smaller main duct on each side, connected by 

 a transverse vessel on the hinder side of each proglottis. 

 In the last proglottis these vessels open by a median pore. 

 The nervous system consists of a ring in the head with 

 small forward nerves and two lateral nerve cords. The 

 reproductive organs have the same general structure as in 

 the liver fluke : they are shown in Fig. 129. A complete set 

 of them is found in each proglottis. Apparently each 



