THE SPONGE-CRAB. 249 



These are placed at the outlet of the stomach, so that the aliment, 

 after being subjected to the action of the jaws, is again more 

 perfectly comminuted by the stomach-teeth before entering 

 the digestive tube. The different pieces composing the 

 masticatory apparatus of the stomach vary considerably in 

 the different genera, and even in the several species of the 

 came genus; but in every case they are always singularly in 

 harmony with the kind of food taken and the general habits oi 

 the animal. 



To enable the decapods to seize their victims or to defend 

 themselves against their enemies, their anterior thoracic extremi- 

 ties generally assume the form of " chelae," claws, or pincers 

 uf considerable strength, armed with teeth or sharp hooks, which 

 give them increased powers of prehension. This form results 

 mainly from the state of extreme development in which the 

 penultimate articulation frequently occurs, and its assumption 

 of the shape of a finger by the prolongation of one of its in- 

 ferior angles. Against the finger-like process thus produced, 

 which is of great strength, and quite immovable, the last 

 articulation can be brought to bear with immense force, as it 

 is put into motion by a muscular mass of great size, and in 

 relation with the extraordinary development of the penultimate 

 articulation. In most cases only the first pair of legs is con- 

 verted into these formidable weapons, but in the Dromias, which 

 are very common in the warmer seas, 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 warfare, but merely for 

 strategical purposes, as they serve to hold roima u gan3 ' 



fast the pieces of sponges, shells, medussp, and other marine 

 productions, under whose cover the wily robber approaches and 

 entraps his prey. 



While the lower crustaceans abound in the polar seas, 

 the crabs are completely wanting in those desolate re- 

 gions ; their number increases with the warmer temperature 

 of the waters, and attains its maximum in the tropical 

 zone. Here we find the most remarkable and various 

 for ms , here thev attain a size unknown in our seas ; and 



