THEIR PHYSIOLOGICAL TALUE. 



21 



ornament the outer edges of the skin- scales of reptiles or the 

 carapace of crabs. Hence we are justified in regarding the 

 sculpture thus produced on th^ epidermis of these animals as 

 a rudimentary organ ; for the microscopic casting hairs, after 

 they have done their duty in. inducing the casting, remain 

 where they were formed, somewhat altered in form, it is true, 

 and without any further visible use. 



Occasionally, however, these hairs, after they have fulfilled 

 their office, are transformed into organs which are capable of 

 serving other useful purposes to the reptile. Thus, for instance, 

 Yon Ley dig discovered certain organs in the skin of reptiles, 

 which he designated as organs of a sixth sense, regularly 

 communicating with long elastic hairs which project far above 



FlO. 9. Stages of castirg in the carapace of the freshwater Cray-fish, from Brann. 

 I. First stage ; a, the two old cnticular layers; 6, the layer of casting hairs ; r, the epi- 

 dermis cells. II. Secoud stage ; a, b, c, as in I. ; between 6 and c the new cuticle d has 

 intervened. 



the surface of the skin, and seem admirably adapted to transmit 

 every impact or molecular movement to the sensitive and 

 guiding organs that are connected with them. These sensitive 

 hairs belong to those casting hairs just mentioned, of which some 

 f -w, that are placed in suitable positions, have been transformed 

 into such sensitive hair-organs (fig. 8, sft). Many of the teeth 

 and ridges which are formed on the surface of the scales by 

 the coalescence of casting hairs are so placed as to be of use in 

 the difficult process of stripping off the whole skin on whose 

 surface they are situated, for they serve as holdfasts to the 

 rough surface of stones. Another still more striking example 

 is exhibited by the family of the Geckos, which are all dis- 

 tinguished by having an immense number of long stiff movable 



