1604 THE STORY OF THE UNIVERSE 



thus obtains the skeleton of a cylinder. The largest 

 holes are filled up with detritus of all kinds. Then 

 these materials are agglutinated by a special secre- 

 tion. The larva overlays the interior of its tube with 

 a covering of soft silk which renders the cylinder 

 water-tight and consolidates the earlier labors. The 

 insect is thus in possession of a safe retreat. Resem- 

 bling some piece of rubbish, it completes its meta- 

 morphosis in peace, undisturbed by the carnivora 

 of the stream. 



Between the beings whom nature has endowed with 

 a shelter and those who construct it by their own in- 

 dustry, we may intercept those who, deprived of a 

 natural asylum and not having the inclination or the 

 power to make one, utilize the dwellings of others, 

 either when the latter still inhabit them, or when they 

 are empty on account of the death or departure of the 

 owner. 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 (Pinnoteres pisum). 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 indolent friend keeps 

 him acquainted with what is passing on around, and 

 as he is much more active and alert than his com- 

 panion he sees danger much further away, and gives 

 notice of it, asking for the door to be shut by lightly 

 pinching the mussel's gill. 



For birds like the cuckoo and the Molothrus it 



