52 PLATYHELMINTHES TREMATODA CHAP. 



Vertebrate. Some Trematodes lodge in the mouth ; others wander 

 down the oesophagus into the stomach or intestine, where they 

 fix themselves to the mucous membrane. Again, others work 

 their way into the digestive glands by the ducts, and thus 

 become further and further removed from the external world, 

 and more adapted to live in the particular organs of that host 

 in which they best flourish. The most important result of the 

 adoption of this internal habitat by endoparasitic Trematodes 

 is, however, seen in their life-history. If a liver-fluke were to 

 deposit its million or so of eggs in the bile-ducts of the sheep, 

 arid these were to develop in situ, the host could not withstand 

 the increased drain upon its vital resources, and host and para- 

 sites would perish together. Hence it is clear that the infection 

 of a second host by Trematodes is highly necessary, whether 

 they be ectoparasitic, in which case the infection is easily effected, 

 when two hosts are in contact, by the adult worms, as well as when 

 they are apart, by free-swimming larvae. In endoparasitic Trema- 

 todes it is brought about by the migration of the young to the outer 

 world, their entrance into a, usually, Invertebrate host and their 

 asexual multiplication within it, and the capture and deglutition 

 of this " intermediate host " by the final Vertebrate one. Within 

 the latter the immature parasites find out the organ in which their 

 parents flourished, and here they too grow and attain maturity. 

 The chances of any one egg of an endoparasitic Trematode pro- 

 ducing eventually an adult are, therefore, far less favourable 

 than in the case of an ectoparasitic form. In other words, while 

 the former must lay a great number of small eggs, the latter 

 need only deposit a (comparatively) few large ones, and this fact 

 has a corresponding influence on the structure of the genitalia 

 in the two cases. The Digenea, which employ two hosts in a 

 lifetime, have accordingly a different generative mechanism from 

 that of the Monogenea. The great need of the latter is a power- 

 ful apparatus for adhering to the surface of the body of its host ; 

 while the adaptations which the endoparasite requires are, in 

 addition, (1) protection against the solvent action of the glands 

 of its host, (2) the power of firm adhesion to a smooth internal 

 surface, and (3) the ability not only to produce a large quantity 

 of spermatozoa and ova, but in the absence of a fellow -parasite, 

 to fertilise its own ova ; and we find these conditions abundantly 

 satisfied. 



