200 MORPHOLOGICAL DEVELOPMENT. 



marked. As shown in Figs. 267 and 268, the contrast 

 between the upper and under parts is greater, and the head 

 and tail ends differ more obviously. In all the higher 



Arthropoda, the unlikeness between the front half and the 

 hind half has become conspicuous. There is in them single 

 bilateral symmetry of so pronounced a kind, that no other 

 resemblance is suggested than that between the two sides. 

 By Figs. 269 and 270, representing a decapodous crustacean 

 divided longitudinally and transversely, this truth is made 

 manifest. On calling to mind the habits of the 



creatures here drawn and described, it will be seen that 

 they explain these forms. The incidence of forces is the 

 same all around the Earth-worm as it burrows through the 

 compact ground. The Centipede, creeping amid loose soil or 

 debris or beneath stones, insinuates itself between solid sur- 

 faces — the interstices being mostly greater in one dimension 

 than in others. And all the higher Annulosa, moving about 

 as they do over exposed objects, have their dorsal and ventral 

 parts as dissimilarly acted upon as are their two ends. 



One other fact only respecting annulose animals needs to 

 be noticed under this head — the fact, namely, that they 

 become unsymmetrical where their parts are unsymmetric- 

 ally related to the environment. The common Hermit-crab 

 serves as an instance. Here, in addition to the unlikeness of 

 the two sides implied by that curvature of the body which 

 fits the creature to the shell it inhabits, there is an unlikeness 

 due to the greater development of the limbs, and especially 

 the claws, on the outer side. As in the embryo of the 

 Hermit-crab the two sides are alike ; and as both the embryo 

 and the ancestor lived in such a way, being free, 

 that the conditions were alike on the two sides ; 

 and as the embryo may be taken to repre- 

 sent the type from which the Hermit-crab has 

 been derived; we have in this case evidence 

 that a symmetrically-bilateral form has been 

 zii moulded into an unsymmetrically-bilateral 



