THE GENERAL SHAPES OF ANIMALS. 183 



more obviously. In all tlie liigher Articidata, tlie 



unlikeuess between the front lialf and the hind half has 

 become conspicuous : there is in them single bilateral 

 Bpnmetry of so pronounced a kind, that no other resem- 

 blance 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. Tiie 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 unsymmetrically 

 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 the embryo may 

 be taken to represent the type from which the 

 Hermit-crab has been derived ; we have in 

 this case evidence that a symmetrically~bi- 

 latcral form has been moulded into an unsvm- 

 metrically-bilateral form, by the action of un- 

 B}Tnmetrically-bilateral conditions. A further 

 illustration is supplied by Bopyrus, Fig. 271 : Zli 



a parasite the habits of which similarly account for its dis- 

 torted shape. 



\h A riciHof/^n 



