182 ON CRABS AVTTICTT CLTXG. 



the limbs as radii of a circle. These modifications have gone 

 much furtlier in Porcellana ; the body is shorter and round, 

 and actually the centre of the circle whose radii are the legs, 

 which are peculiarly effective grapjjlers, as you may see by 

 alloAving the animal to hang on to your hand. A further 

 consequence of tlie clinging habit is a very conspicuous flat- 

 tening, a feature noticeable in so many animals in almost any 

 group when they have taken to live on the under side of 

 stones. I need only mention here the flattened spiral of the 

 ormer, the flat-jointed shell cover of Chiton, the " Flat 

 Worms," almost reduced to a mere plane surfac^e, Poly nee and 

 other Annelids which are so much broader than their relatives 

 who swim or crawl through sand and mud. 



The advantage of this flattening is obviously a great one, 

 it reduces the disloflgiiig power of a side blow or wash. In 

 PorceUana jAatyclLclea too, the mud-coloiu'ed hairs covering 

 the body no doubt mitigate the dangers of a wash from the 

 side by splitting and deadening the force of the sweep just as 

 it reaches the animal. These hairs, however, have other and 

 more important functions, the chief being to increase the 

 resemblance of the animal to its surroundings. Those with 

 more and more hair seem to have survived age after age 

 through avoiding some enemy or enemies which must hunt by 

 sight. Perhaps fish may be adjudged the guilty parties here, 

 though Octopus need not be innocent, while prawns and their 

 relatives have been seen in my tanks devouring the remains of 

 a PorceUana, which, however, they may not have killed. 



Mr. Sinel has noticed an extreme effect of the clinging 

 habit. Not only do the body and limbs crowd into a very 

 small space as usual in such circumstances, but the crowding 

 is so great that the inner side of the great claw itself is con- 

 cave, elsewhere it is always convex, to allow it to fit exactly 

 against the antero-lateral part of the body. 



When once warned, in fact, PorceUana is very hard to 

 detach from the stone to which it clings, and there is no doubt 

 that, both in this direction and in the direction of protection 

 from sight-hunters, adaptation has been carried to extremes. 



PorceUana longicornis offers a marked contrast ; it has a 

 much longer and thinner great claw and the body is practically 

 free from hairs of any size, so that here we must have a con- 

 siderable contrast of habit between two species of one and the 

 same genus. It is noteworthy that Jonyicornis scuttles away 

 when disturbed, using his long great claws sometimes to push 

 his body along, while platycheles, as was explained, hangs 

 on with all his might. 



