138 



MANUAL OF ZOOLOGY 



SECT. 



a proboscis. The mouth is never at the extreme anterior 

 end, but is always ventrally placed, sometimes behind the 

 middle. A few multiply by budding, and these may give 

 rise to chains of individuals, which subsequently become 



separated. In the lowest Tur- 

 bellaria the intestine is repre- 

 sented merely by a nucleated 

 mass of protoplasm ; in others it 

 is a simple sac ; in the major- 

 ity it is branched. The general 

 structure of the other internal 

 organs very closely resembles 

 that of the corresponding parts 

 in the Trematodes. 



FlG. 73. Planarapolychroa(a), Turbcllaria OCCUr in the SCa, 

 Ivgubris (&}, torva (c), about , . j i j 



thrice the natural size. (After in fresh water, and also in damp 



Schmidt, from Claus.) , , . . , , .,. 



localities on land. The great 



majority are non-parasitic, their food consisting of minute 

 aquatic animals and plants of various kinds. An example 

 is Planaria torva of our fresh-water pools and streams 

 (Fig. 73,6-). 



3. THE CESTO 



The class Cestoda or tape-worms ^Kill internal parasites, 

 and in the adult condition live in the enteric canal of verte- 

 brates. The tape-worms are much more completely adapted 

 to a life of parasitism than the Trematodes : they have no 

 digestive system, and are nourished by the imbibition, 

 through the general surface, of liquid nutriment derived 

 from the digested food of the vertebrate host. The shape 

 of a typical tape-worm is widely different from that of a 

 trematode. A tape-worm (Fig. 74) is flattened like a 

 trematode, but is extremely elongated, the length being 



