PHYLUM PLATYHELMINTHES 161 



The great number of ejj^s ])roduced by a single fluke is neces- 

 sary, because the majority of the larva- do not find the particular 

 kind of snail, and the cercariae to which the successful larvae 

 give rise have little chance of being devoured by a sheep. The 

 generations within the snail of course increase the number of 

 larvae which may develop from a single egg. This complicated 

 life-history should also be looked upon as enabling the fluke to 

 gain access to new hosts. The liver-fluke is not so prevalent 

 in the sheep of this country as in those of Europe. 



b. Trematoda in General 



The TREMATODA are parasitic Platyhelminthes without cilia 

 but with a hardened ectoderm in the adult stage. The body is 

 usually flattened and leaf-shaped. One or more ventral suckers 

 are present at or near the posterior end and in the mouth region. 



Trematodes may be ectoparasitic, i.e. living on the body of 

 another animal, like Gyrodactylus which clings to the gills of the 

 carp, or ento parasitic, i.e. living in the body of another animal, 

 like the liver-fluke. Some of the modifications due to parasitic 

 habits are the absence of eye-spots in most species, the poorly 

 developed brain and sense-organs, and the highly specialized 

 sexual organs. 



The two orders of TREMATODA differ principally in their method 

 of development. 



Order i. Monogenea. Trematodes which develop directly 

 from the egg; they possess a large posterior, ventral, terminal 

 sucker, and usually one or two suckers near the mouth. 



Most of the MONOGENEA are ectoparasitic on aquatic animals, 

 e.g. SpJiyra ultra on the skin of the salamander (Necturus), 

 Polystomum on the gills of the tadpole and later in the urinary 

 bladder of the adult frog, and Epibdella on the body of the 

 halibut. 



Order 2. Digenea. Entoparasitic TREMATODA which pass 

 through several different forms in their life-history; they pos- 

 sess an anterior and often a ventral sucker. 



M 



