SEA-SPIDERS. 185 



vided with smaller pincers, as a kind of reserve, in case the first 



pair should be rendered unserviceable ; and 



in the Dromise we find the two posterior 



pairs of legs, which are of a much smaller 



size and raised above the plane of the others, 



similarly armed. These posterior claws, 



however, are not intended for active war- 



fare, but merely for strategical purposes, as 



they serve to hold fast the pieces of sponge or other marine 



productions under whose cover the wily crustacean approaches 



and entraps his prey. 



A singular Tahitian crab observed by Mr. Bennett makes use 

 of a similar artifice; but in this case the mask of decayed 

 vegetable substances and coral sand, which enables the lurking 

 ruffian to steal upon his victims unperceived, is not kept in its 

 position by the hind-legs, but by the rigid and incurved bristles 

 with which the back is covered. In these manoeuvres he is very 

 much assisted by the long ophthalmic peduncles, which, curving 

 upward to raise the eyes above the pile of materials, give him 

 the great advantage of seeing without being seen. 



The sea-spiders do not indeed load themselves with a volun- 

 tary burden, which they are able to cast off again at pleasure, 

 but their back is generally clothed with a mass of parasites, 

 corallines, sponges, zoophytes, algae or molluscs, so that the 

 poor creatures have frequently to groan under a considerable 

 weight. To this, however, they are no doubt in many cases 

 indebted for their lives, as even a sharp-sighted enemy can 

 hardly detect them under the mound of plants and small 

 animals comfortably settled on their carapace. Besides the 

 strong and heavy pincers with which the forefeet of the Birgus- 

 latro are armed, this large tropical shore-crab has, like the 

 Dromiae, its last pair of legs terminated by narrow and weak 

 claws, which, however, it puts to a very different use, for, living 

 on the fruits of the cocoanut-tree, it requires no mask or artifice 

 to surprise its prey. After having selected a nut fit for its dinner, 

 the crab begins its operations by tearing the husk, fibre by fibre, 

 from that end under which the three eyeholes are situated ; it 

 then hammers upon one of them with its heavy claws until an 

 opening is made. Hereupon it turns round, and by the aid of 

 its posterior pincers extracts the white albuminous substance. 



