TRANSMISSIBILITY OF FUNCTIONAL MODIFICATIONS 



105 



body up to the hard claws when necessity arises. In this case there 

 is surely a continual pressure on the abdomen, which, being soft, 

 must be squeezed very tightly every time the animal retreats into 

 its shell. One of my opponents has described the disappearance of 

 the tough integumentary skeleton from the abdomen of these animals 

 as an inherited result of this pressure, and another regards it as the 

 inherited result of the degeneration of the muscles in this part of 

 the body. But, according to Zehnder, this continuous pressure, and 

 the frequent rubbing up and down of the abdomen on the inner 

 surface of the Gasteropod's shell, would undoubtedly have a stimu- 

 lating efi'ect on the skin-nerves, and would therefore bring about 

 a thickening of the chitinous cuticle. In regard to the larval 

 Phryganidae and Psychidse, the case would be the same, though 



Fig. 107. Larva of a Caddis-fly, after ROsel. A, removed from its case, 

 showing the hooks (A) which attach it thereto, and the whitish abdomen, 

 covered only by a thin cuticle. JB, the same larva, moving about with its case. 



perhaps hardly to the same degree, for while these larvae make 

 their own houses, and will therefore at least make them big enough 

 to begin with, the pressure and friction must increase with the 

 growth of the animal. 



If the regulation of the strength of the integumentary skeleton 

 be referred to selection, we see at once why carapace and wing-covers 

 should be of equal thickness throughout their whole extent, and why 

 they do not disappear, although they do not function actively, and 

 are less stimulated than any other parts of the skeleton ; and we 

 also understand why the abdomen of hermit-crabs and of larval 

 Phryganidse and Psychidse has become soft, whether it be exposed 

 to pressure or friction in a greater or a less degree. It no longer 

 requires to be hard, because it is protected by the house, and in 



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