TRACHEATES 205 



from the skin by secretion, it could not be altered by- 

 use, because it was the old, overlying shell that was 

 then in use. The characteristics of the articulate 

 skeleton cannot, therefore, have been developed by use, 

 as in each case they were fully developed before use. 

 When a crab casts its old coat, it appears in the new 

 one, which is still soft, but soon sets. But the soft 

 shell has all the protuberances and thick parts, and does 

 not differ in the least in structure from the hard one. 

 Moreover, it would not help even if the soft shell could 

 be modified by use. The soft crab refrains from using 

 his soft claws, with which he could do nothing. He 

 creeps under a stone and waits idly until the coat is firm 

 enough to protect him. 



To this view it has been replied as follows. The 

 shell may not be able to thicken from pressure, but the 

 skin that is forming the new coat underneath it may. 

 Pressure in the old shell affects the underlying skin as 

 well, and its function, the secretion of a new shell, will 

 be proportionately stimulated, and produce it in a 

 thicker condition. Hence when the crab continually 

 uses its claws and so presses on the inner side of them, 

 this pressure will act through the shell on the skin 

 beneath it ; this will do more work, and at the next cast 

 of the coat the inner edge of the claws will be thicker. 



But why should the skin act more and not less 

 vigorously under pressure ? It is not at all agreed that 

 it secretes a thicker and not a thinner coat, when it is 

 pressed through the overlying shell. And even if we 

 admit that its activity is increased by pressure, how 

 did this capacity come into existence.'' 



