248 THE INITABITANTS OF THE SEA. 
The Thoracostraca are subdivided into the small group of 
the Stomatopoda, whose branchiz are external and the feet 
prehensile or formed for swimming, and 
the far more numerous and iniportant 
Decapods, which are either long-tailed 
like the scyllarus or short-tailed like the 
Scyllarus equinomalis. crab. In these the branchie no longer 
float in the water, but are enclosed in 
two chambers, situated one at each side of the under surface 
of the broad shelly plate which covers the back of the animal. 
Each of these chambers is provided with two apertures, one in 
the front near the jaws, the other behind. 
The disposition of the anterior or efferent orifice varies but 
little; but in the long-tailed species the afferent or posterior 
orifice is a wide slit at the basis of the feet, while in the short- 
tailed kinds it forms a small transverse aperture generally 
placed almost immediately in front of the first pair of ambulatory 
extremities. By means of this formation, the short-tailed de- 
capods or crabs, like those fishes that are provided with a narrow 
opening to their gill covers, are enabled to exist much longer 
out of the water than the long-tailed lobsters. Some of them 
even spend most of their time on land; and, still better to adapt 
them for a terrestrial life, the internal surfaces of the branchial 
caverns are lined with a spongy texture, and the gill branches 
separated from each other by hard partitions, so as to prevent 
them from collapsing after a long penury of water and thus 
completely stopping the circulation. While in fishes the water 
that serves for respiration flows from the front backwards, so as 
not to impede their motions, we find in the interior of the 
branchial cavity of the decapods a large valve attached to the 
second pair of maxillary feet, which, continually falling and 
rising, occasions a rapid current from behind forwards in the 
water with which the cavity is filled, a structure perfectly 
harmonising with their retrograde or sidelong movements. 
The digestive apparatus of the decapods is of a very com- 
plicated structure. The mouth is here furnished with at least 
eight pieces or pairs of jaws, which pass the food through an 
extremely short gullet into a stomach of considerable size. This 
stomach is rendered curious by having within certain cartilagi- 
nous appendages, to which strong grinding-teeth are attached. 
