138 



MANUAL OF ZOOLOGY 



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 

 tt ° jj? in the Trematodes. 



Fig. 73. — pianaria poiychroa (a), Turbellaria occur in the sea, 



lugitbris (<S), torva (c) , about - , 1 , ■ 1 



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



Schmidt, from Claus.) .... . . „,. 



localities on land. ihe 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, c) . 



3. THE CESTODA 



The class Cestoda or tape-worms are all 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 



