192 DECAPOD CRUSTACEA. PART in. 



jaws, followed by three pairs of foot-jaws ; so called 

 because they are legs modified to serve as jaws, but in 

 some crustaceans they are also instruments of loco- 

 motion or prehension, and sometimes of both. The 

 two last pairs have palpi, or feelers, at their base. All 

 the jaws and foot-jaws, when not in use, are folded over 

 the mouth ; the joints of the two last are so broad that 

 they completely conceal this complicated apparatus. 



Posterior to the mouth and its organs there is a flat 

 broad plate, which forms the ventral side of the body, 

 with a groove in its surface, into which the rudimentary 

 tail is folded back, as in the Carcinus mosnas (D, fig. 

 148), and the feet are fixed by movable joints on each 

 side of this sternal plate. The first pair, which are a 

 little in advance of the others, and bend forwards in a 

 curve towards each other, may be called hand-feet, as 

 they occasionally serve for both. They have very thick 

 short arms and swollen hands, having a curved finger 

 and a thumb with a movable hinge, armed throughout 

 their internal edge with a row of blunt teeth, and ter- 

 minated by sharp points. The other four pairs, which 

 are the real walking feet, spread out on each side of 

 the animal, and often bend a little backwards ; they are 

 rarfcher thin, compressed, and end either in a horny nail, 

 or flattened blade for swimming. 



The gills, which are the breathing organs of the crabs 

 and other Decapods, are spindle-shaped bundles of long, 

 slender, four-sided pyramids, fixed by their points on 

 each side of the mid line of the throat, so that they ex- 

 tend in opposite directions, and their spreading bases 

 fit and rest upon the vaulted sides of the carapace, or 

 rather gill chambers, to the right and left. Each of the 

 pyramids is formed of a multitude of parallel membra- 

 nous cylinders fixed to the axis of the pyramid, and 

 an infinity of capillary bloodvessels form a network in 

 their surfaces. 



The crab has nine of these bundles of gills in each 



