HERMIT-CRABS 213 



take these as an example, of course develop in the body of 

 the mother, and the embryos possess an inherited light 

 covering of hair, which is no longer present in the adults. 

 That the action of water is unfavourable to the cornification 

 of the epidermis necessary for the formation of hair — be it 

 even merely in the succession of the hairs during the later 

 independent life — and that the hair, therefore, and also be- 

 cause it could be dispensed with and was not an advantage 

 to the motion of the animal, has been gradually lost, is not 

 surprising. Cornification depends essentially on the loss of 

 water. Only exceptionally, for particular purposes, is horn 

 developed here and there in exclusively aquatic animals 

 {e.g. the horny teeth of the lampreys, whalebone of whales). 



On the other hand, we find cuticular structures — hard 

 secretions formed by the epidermis — widely distributed in 

 aquatic animals, and forming their egg-cases. Such cuticular 

 structures often form a case for the body of the animal, into 

 which it can withdraw itself. This secretion consists of 

 other material than the cornified epidermis of the verte- 

 brates. 



The parts of the body which are included in such cuticular 

 sheaths often degenerate. Similarly, the hinder part of the 

 body of the hermit crab, which is inserted into a whelk shell, 

 degenerates, and the hard cuticle of the crab which is 

 protected by the shell has become soft and tender. Weis- 

 mann ascribes this alteration entirely to the absence of 

 natural selection. He says : " The use of the shell depends 

 sim^Dly on its entirely passive presence. Whether the animal 

 is by it protected against thrusts or bites, or whether it is 

 threatened by no such dangers, is quite indifferent to the 

 shell itself and its proper development ; it loses and gains 

 nothing thereby, and least of all does its perfection depend 

 on its being as often as possible battered with thrusts and 

 bites. It cannot possibly be caused to atrophy by the fact 



