INTRODUCTION TO ZOOLOGY 



alimentary canal. Its oval body is about half an inch long, 

 and is pointed at both ends. It lives in the water, gliding 

 over the water-weeds in search of the small worms and insect 

 larvae on which it feeds. 



The Trematoda are Flatworms which, unlike 

 Li^CT-flukes Tiirbellaria, are parasitic, and are not usually 

 ciliated. The body is provided with suckers 

 with which the Trematod clings to its prey. The Liver- 

 fluke of the sheep {Distomum kepaticum) is one of these. 

 It has an extraordinary life-history, living part of its life 

 inside the liver of a sheep, and the other part within the 

 body of a water-snail (Limnaea truncatula), or of a land-snail 

 (Helix). It passes out from the snail on to the grass, and is 

 then swallowed by the sheep when it eats the grass. ^ 



The Tapeworms are also parasitic ; they have 

 Tapeworms. * ^°"S' ribbon-like, segmented body with no ali- 

 mentary canal. The segments are very little 

 dependent on each other ; each is capable of reproduction, and 

 separates from the rest when ripe, leaving the body of its 

 host, and passing the second stage of 

 its life in another host. 



Taenia solium, the tapeworm para- 

 sitic in man, passes the second stage of 

 its life in the pig as the " bladder 

 worm," and then again infects man if 

 the pig's flesh is eaten without having 

 been previously cooked sufficiently to 

 kill the bladder worms.^ 

 Nemertinea. The Nemertine worms 

 Ribbon- are long, soft-bodied, unseg- 

 worms. mented forms, probably 

 nearly related to the whirl-worms. 

 Like them, they have no body-cavity, 

 and they have a covering of fine cilia. 

 Fig. 50.— Idneus marinus. \yxxt in some other ways they are more 

 ^NMuralHist^^S^'^' !»ig^V developed ; specially character- 

 . ^ . , , ^ . istic of them is the long muscular pro- 



a, Anterior end ; 6, posterior . nir ■, 



end. boscis which can be protruded from the 



front end of the body just above the 



mouth. It has, however, no connection with the mouth, 



^ For full life-history see Parker and Haswell's Zoology, pp. 226-37. 



