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 
same genus; but in every case they are always singularly in 
harmony with the kind of food taken and the general habits of 
the animal. 
To enable the decanods to seize their victims or to defend 
themselves against their enemies, their anterior thoracic extremi- 
ties generally assume the form of “chelx,” claws, or pincers 
of 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 Dromiz, 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 
fast the pieces of sponges, shells, medus@#, 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 
forms, here they attain a size unknown in our seas: and 
Dromia Vulgaris. 
