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 Turbellaria the intestine is represented merely 

 by a nucleated mass of protoplasm : in others it is a simple 

 sac ; in the majority it is branched. The general structure 

 of the other internal organs very closely resembles that of the 

 corresponding parts in the Trematodes. 



Turbellaria occur in the sea, in fresh-water, and also in 

 damp localities on land. The great majority are non- 

 parasitic, their food consisting of minute aquatic animals and 

 plants of various kinds. 



3. THE CESTODA 



The class Cestoda or Tape-worms are all internal para- 

 sites, and in the adult condition live in the enteric canal of 

 Vertebrates. 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. 68) is flattened like a 

 Trematode, but is extremely elongated, the length being 

 many times, often hundreds of times, the greatest breadth, so 

 that the animal assumes the form of a long narrow ribbon or 

 tape. This ribbon is not continuous, but is made up of a 

 string of segments or proglottides. Towards one end the 

 body becomes narrower, terminating in a rounded knob 

 the head or scolex. On the head (Fig. 69) is a circlet of 

 hooks borne on a rounded prominence, the rostellum^ 



