134 MANUAL OF ZOOLOGY sect. 



head-lobe {pap). There is no vestige of internal organs, with 

 the exception of a pair of flame-cells. The ciliated larva swims 

 about in water, or moves over damp herbage for a time, and 

 perishes unless it happens to reach a pond-snail (Limnaa) , 

 as a parasite of which it is alone able to enter into the next 

 phase of its life-history. When it meets with the snail, 

 destined to form the second or intermediate host of the 

 parasite, the embryo bores into it by means of the head- 

 lobe. Established in the interior, it grows rapidly into the 

 form of an elongated sac, the sporocyst (Fig. 70, B), with an 

 internal cavity. Eventually cells are budded off from the 

 interior of the sporocyst, each of which gives rise to a body 

 called a redia (C). When fully formed the redia is a 

 cylindrical body, having a mouth leading to a pharynx, fol- 

 lowed by a simple sac-like intestine, and a system of excre- 

 tory vessels. The rediae, after escaping from the interior of 

 the sporocyst, bud off internally cells which either give rise 

 to a fresh generation of redise or to bodies termed cercaritz. 

 The latter (D) are provided with long tails, with anterior 

 and posterior suckers, a mouth and pharynx, and a bifid 

 intestine. These escape through an aperture in the wall of 

 the redia, and, moving actively by means of their tails, force 

 their way out from the body of the snail. They then, losing 

 the tail, become encysted, attached to blades of grass or 

 herbage. The transference of the larval fluke to its final 

 host, the sheep, is effected if the latter swallow the grass on 

 which the cercaria has become encysted. The young fluke 

 then escapes from the cyst, and forces its way up the bile- 

 ducts to the liver, in which it rapidly grows, and, developing 

 reproductive organs, attains the adult condition. 



The liver-fluke is an example of the class of flat-worms 

 known as Trematoda. These are all parasitic. Some are 

 internal parasites, and in the adult condition inhabit, for the 



