44 THE INDUSTRIES OF ANIMALS. 



from their expeditions with pouches filled with fish, 

 the Caracaras attack them until they disgorge, and 

 then alight to devour the stolen prey. They do not 

 attack the outgoing birds, but only the incoming 

 ones, and they wait until they reach the land (so that 

 the contents of the pouches may not fall into the 

 water) before pouncing on them.^ 



Among other animals a habit has been formed from 

 some special circumstance. As an extreme case in 

 this group we meet with parasites of whom some 

 cannot live outside a particular nest, and are even 

 absolutely transformed by this kind of life. But 

 between these and independent hunters there are 

 an extreme number of intermediate stages, of which 

 it is sufficient to mention a few.^ 



The Fierasfer, a little fish of the Mediterranean, 

 installs himself in the respiratory cavity of a Holo- 

 thurian ; he does not live at the expense of his host's 

 flesh, but contents himself with levying a tax on the 

 foods which enter the cavity. It is a case of com- 

 mensalism of which there are very numerous ex- 

 amples. Other cases may be mentioned which are 

 still further removed from parasitism. Among these 

 may be mentioned the birds who relieve large 

 mammals of their vermin. 



One of them, the Red-beaked Buffalo bird {Buphaga 

 erythrorhyncha), lives in Abyssinia. This bird is in- 

 sectivorous. He has remarked that the ruminants 

 constitute baits for flies; therefore he never leaves 

 these animals, hops about on their backs and delivers 

 them from annoying parasites ; the buffaloes, who 



1 Bendire, Life Histories of North American Birds, p. 315. 

 ''■ For a discussion of this subject, see P. van Beneden, Commemaux 

 ei Farasites, Paris, 1875. 



