160 ANIMAL PARASITES AND MESSMATES. 



what certain groups of animals are able to accomplish. 

 The crustaceans everywhere perform the office of vul- 

 tures to clear the waters from dead bodies, whether large 

 or small, and they are in general sufficiently numerous 

 to perform this police duty effectually. We may say 

 that without their aid the waters along the coasts and 

 at the mouth of rivers would grow speedily corrupt and 

 unfit to support life. Thus it sometimes happens that 

 when the number of these beings is insufficient, or the 

 putrescible matter is in excess, we see the fish, the mol-. 

 luscs, and even the crustaceans, perish one after the 

 other. 



The last of the parasites of this category are known 

 by the name of Gregarinae. It appears that Goede. was 

 the first to make observations upon them. Leon Dufour 

 gave them the name which they still bear. They have a 



Fig. 33. — Greqarina of Nemerte* Fig. 34. — Sac -with PsoTospenniro 



Gesseriensig. from the Sepia officinalis. ' 



very simple organization, and are formed only of a cell 

 which contains a nucleus : they live in the intestines of 

 many invertebrate animals, especially in the articulata. 

 Let us imagine a body, long, more or less transparent, 

 with a smooth surface very like a spindle, which glides 

 about in the intestines, in the midst of the liquid matter 

 which it contains, without our being able to ascertain 



