VII.] 



THE EXTERNAL SKELETON. 



239 



the arms are then drawn out completely inverting the ecte- 

 ronic investment of each separate finger. It is then pushed 

 back over the loins, and the legs are withdrawn, and also the 

 feet and toes, the skin being inverted just as a glove is turned 

 inside out. Finally the tail is pulled out, and then the whole 

 structure being rolled up in a mass is swallowed at a gulp. 

 It is easy, with a little patience, to shake out one of these 

 skin- casts in water, so as to produce a complete and as it 

 were shadowy image of the little creature that bore it. 



The epidermis is never converted into bone, but is often 

 thickened and converted into horn, an approach to which 

 we may see in man in the labourer's hand, in the sole of the 

 foot, and in those unwelcome deposits on the toes corns. 

 " Horn," indeed, is but a thickened form of the very same 

 material as that of which the minute particles we shed from 

 the outer surface of our skin is made, and which yield the 

 substance " gelatine " when boiled. Certain local thickenings 



FIG. 203. DORSAL SURFACE OF THE CARAPACE OF A FRESH-WATER 



TORTOISE (Ewys). 



i 8,"expanded neural spines ; r 1 ^, expanded ribs ; , first median (or nuchal) 

 plate ; py, last median (or pygal) plate ; in, marginal scutes. The dark lines 

 indicate the limits of the plates of the horny epidermal tortoise-shell ; the thin 

 sutures indicate the lines of junction of the bony scutes. 



may exist in animals, not abnormal like those above referred 

 to in man, but constant in each species. Such thickenings 

 are termed callosities, and may exist on the inner side of 

 the legs, as in the Horse, or on the breast, as in the Camel, 



