142 THE INDUSTRIES OF ANIMALS. 



is more or less closely related to a group which 

 appears at first of an entirely different nature. Thus 

 it does not enter into our plan to speak of parasites. 

 Yet, if among these some turn to a host to demand 

 of him both food and shelter, if even they can come 

 to be so modified and so marked by parasitism that 

 they can live in no other way, there are others who 

 ask for lodging only from an animal better protected 

 than they are themselves. It is these whose customs 

 we are called upon to consider. In the interior of the 

 branchial chamber of many bivalvular Mollusca, and 

 especially the Mussel, there lives a little crustaceous 

 commensal called the Pea-crab {Piiinoiercs pisuni). 

 He goes, comes, hunts, and retires at the least alarm 

 within his host's shell. The mussel, as the price of its 

 hospitality, no doubt profits by the prizes which fall 

 to the little crab's claws. It is even said that the crab 

 in recognition of the benefits bestowed by his indo- 

 lent friend keeps him acquainted with what is passing 

 on around, and as he is much more active and alert 

 than his companion he sees danger much farther 

 away, and gives notice of it, asking for the door to be 

 shut by lightly pinching the mussel's gill. But this 

 gratitude of the Crustacean towards a sympathetic 

 bivalve is merely a hypothesis ; we do not exactly 

 know what passes in the intimacy of these two 

 widely-differing natures. 



For birds like the Cuckoo and the Molotlirus it is 

 not possible to plead attenuating circumstances. 

 They occupy a place in an inhabited house without 

 paying any sort of rent. Every one knows the 

 Cuckoo's audacity. The female lays her eggs in 

 different nests and troubles herself no further about 

 their fate. She seeks for her offspring a shelter which 



