ANIMAL PARASITES 17 



groups of cells of varying size from which develop the next genera- 

 tion, the rediac. 



The redice are characterized by an unbranched saclike ali- 

 mentary canal with a single opening, the mouth. The rediae 

 wander about in the snail, being especially abundant in the liver. 

 Within them may be produced daughter redice or, especially in the 

 fall and winter, the next larval form, the cercariae. 



The cercarice (Fig. 5) are usually tadpole-like in shape and are 

 at first very active. They possess a tail, one or two suckers, a 

 bifurcate alimentary canal with a single opening, the mouth, and 

 the rudimentary genital organs. The excretory system, which 

 consists of a complicated series of "flame cells" and of collecting 

 tubules, will be demonstrated (see p. 16). Its development in 

 one of the holostome cercaria is shown in the accompanying 

 figure from Cort and Brooks, 1928. In any locality there may 

 be found cercaria) of several types which should be determined 

 by the appended key. Occasionally while being studied, a 

 cercaria may be seen to contract, form a clear colorless cyst about 

 its body, and discard its tail, which may continue to thrash about 

 independently for a time. The encysted forms are known as 

 metacercariae. 



The encysted metacercarice occur on vegetation or on, or in, 

 other animals, depending on the species of fluke from which they 

 are derived. Various crustaceans and insects, fish, and frogs 

 are commonly infected and are the intermediaries through which 

 the parasite is passed on to its final host. 



Certain flukes, such as the schistosomes, or blood flukes of man 

 and animals, omit the redia stage in their life cycle, cercariae 

 being produced within the sporocysts. Their characteristic 

 fork-tailed cercariae do not become encysted but bore actively 

 into the skin of the final host. There will be demonstrated 

 Cercaria elvai, a species which is parasitic on some of the lower 

 mammals and is often a cause of transitory skin irritation of man, 

 known as "swimmer's itch." 



