198 



ANIMAL PARASITES AND MESSMATES. 



Any one who wishes to make observations on dis- 

 tomes in the state of cercarise has- only to examine some 

 fresh-water molluscs, either the Limness or Planorbes 

 found in ponds ; as he tears the animal to pieces on the 

 stage of a simple microscope, he will not fail to perceive 

 a multitude of struggling and wriggling tadpoles. Their 

 tails twist with each other, furl up, extend, and describe 

 arcs of circles, as if we had a nest of serpents under our 

 eyes. 



Each species of distome has it own cercarise, which 



are scattered among as many 

 different inferior animals. Birds 

 and fishes become infested by 

 them in consequence of eating 

 these animals. 



We may here cite as an 

 example of this class of para- 

 sites the Distomwn hepaticum, 

 or liver fluke ; this species is the 

 most interesting to us of all the 

 genus ; it attains the size of a 

 moderate leech, and habitually 

 resides in the liver of the sheep. 

 Fig. 42.— Liver flake of twice the In Order to discover it, we have 



natural size ; a, mouth ; b, penis; 



c, digestive tube; d, abdomiual onlv to examme a frcsll livCr. 

 sucker. '^ 



They are usually found in the 

 biliary canals, where they move about like planarise. It 

 is always of a deep colour, and is doubtless introduced in 

 the state of cercaria, when the animal is drinking. M. 

 Willemoes-Suhm supposes that the Distomum liepaticum 

 has for a vehicle a small snail, the Limax agrestisy 

 which the sheep swallows with the grass on which it 



