20 



INTRODUCTION. 



calculated for this end iw evident to me from the fact established 

 by Dr. Braun that the casting of the shells of river Cray-fish 

 is induced in exactly the same manner by the formation of a 

 coating of hairs which mechanically loosens the old skin or 



Fig. 8. — Castinir process in tlae sltin of reptiles, a, in the clinging bristles of tl;e foot 

 of tlie Gecko ; witliin the epidermis are seen the casting hairs, hh, destined to form 

 the new clinging l.ristles. 6, in tlie Adder ; hh, the casting hairs ; the portion of skin 

 above them is pushed away, and the hairs themselves form the ridges on the new skin, 

 c, a scale of Phyl/ociac/t/1'i.s, witli the little sensitive hairs xh .at the right edge, d, Thtca- 

 dat:tylu>i ; .«ft, the sensitive bail's ; and hh, the casting hairs which do not change 

 during casting. From Cartier. 



shell from the new. Now the investigations of Braun and 

 Cartier have shown that these casting hairs — which se:ve the 

 same purpose in two groups of animals so far apart in the 

 systematic scale — after the casting are partly transformed into 

 the concentric stripes, sharp spikes, ridges, or warts which 



