WORMS 71 



and the Trematodes, represented by the Liver- 

 fluke, which infests sheep, together make up the 

 group of flat-worms (Platyhelminthes), of which 

 mention has already been made (p. 44). In all of 

 them the body is more or less flat, and the digest- 

 ive cavity, like that of Coelenterates, has but one 

 opening, the mouth. The life-history of parasitic 

 worms is described in a well-known volume by 

 Leuckart, which forms the basis of our knowledge 

 on the subject. Since its publication, discoveries 

 regarding parasites have been constantly added 

 by other observers. 



The history of the Liver-fluke is a most com- 

 plicated example of alternation of generations. 

 The adult form infests the sheep's liver. There 

 it produces eggs, which afterwards find their way 

 into water. Here they die unless they find their 

 way into a certain water-snail, which many of 

 them do. Within this snail Linntza truncatula 

 the egg develops into a sac-like body, called a 

 sporocyst. This produces within itself numbers 

 of a small creature which is called the Redia form. 

 These in turn produce a tailed form, called a 

 Cercaria, which gets out of the snail, swims in 

 water, and finally settles down on some plant. 

 Here it is eaten by an unfortunate sheep, within 

 which it develops into the adult fluke. 



The other great divisions of the Vermes are 

 as follows : The Nematodes or thread-worms, a 

 group of parasites which includes the dreaded 

 Trichina ; the Nemertines, a group mostly car- 

 nivorous, possessing a curious proboscis, and often 

 an armed skin ; the Leeches or Hirudmea, and 

 finally, the Chsetopods (Bristle-footed Worms), the 

 highest group of all, containing the forms often 

 spoken of as Annelides i.e. Ring-shaped Worms. 



