■248 THE INHABITANTS OF THE SEA. 



The Thoracostraca are subdivided into the small group of 

 the Stomatopoda, whose branchiae are external and the feet 

 prehensile or formed for swimming, and 

 the far more numerous and important 

 Decapods, which are either long-tailed 

 like the scyllarus or short-tailed like the 

 Royiiarna eq-omoxiaiis. crab. In these the branchiae 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 t 

 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 stroDg grinding-teeth are attached* 



