MUTUALISTS. 79 



fessor Semper has informed me, which lodges in the 

 intestines of a fish, with its head usually projecting 

 outwards, and which watches the crustaceans attracted 

 by the excreta of its host ; but although it chooses the 

 intestine of its neighbour as a place of shelter, it is not 



a parasite. 



Fishermen affirm, and the examination of the 

 animal's stomach confirms their assertion, that the 

 Cijclopterus lumpus feeds on nothmg but the excreta of 

 other fishes. Indeed, it is not possible to count the 

 number of intestinal worms known by the name of 

 Scolex, which are found in the contents of the stomach 

 and the intestines. Besides this, we have long known 

 the pecuharities of some insects which cannot live 

 except on the dung of certain animals; and there is an 

 example of one of these insects, found in a fossil state, 

 which anticipated the discovery of the remams ^ of 

 an extinct mammal before unknown in that district. 

 The larvse of the fly Scatophaga stercoraria live only on 

 excrementary matter. 



There are also nematode worms which exist under 

 these conditions, and which develop and propagate their 

 species in the intestines as if in the midst of damp earth. 

 The small eel-like creatures so abundant in cow-dung 

 propagate in it ; they are not parasites, and are aUied to 

 those of w^hich we speak in this chapter. 



Besides those attendants which busy themselves 

 about the cleanlniess of other animals, we find some 

 whose duties are less extensive, and whose cares are 

 more limited. Many animals produce a greater number 

 of eggs than they can bring to perfection, and those 

 which are decomposed for want of fecundation, or which 



