ASTACUS FLUVIATILIS. 279 



diac piece, their inferior ends move downward, backward, and 

 inward, carrying with them the anterior ends of the postero- 

 lateral pieces, the teeth of which (lateral cardiac) come into 

 contact with the urocardiac and cardiac teeth with a force 

 proportional to that exerted in traction. On ceasing to pull, 

 the apparatus returns to its former position, its backward 

 movement being facilitated by the reaction of the elastic 

 pouch mentioned above, and being doubtless also assisted, in 

 the living state, by a pair of small cardio-pyloric muscles, 

 which pass, one on each side, between the cardiac and pyloric 

 ossicles, beneath the membrane of the stomach, the looseness 

 of which, in this region, where it unites the various ossicles 

 of the gastric mill, greatly assists the free movement of the 

 whole apparatus. 



Nothing can be more easy than to perform the experi- 

 ment, and to convince one's self that these structures do really 

 constitute a most efficient masticatory apparatus ; and it is 

 surprising that Oesterlen, in his elaborate essay on the stom- 

 ach of Astacus, should have questioned the crushing action 

 of the teeth. 



A great bilobed valvular process (Fig. 73, c) rises up from 

 the sternal region of the stomach, opposite the cardio-pyloric 

 constriction, and apparently prevents the food from passing 

 into the pyloric division until it is properly comminuted. 

 And, in front of this valve, the infero-lateral parietes of the 

 stomach are strengthened by a number of other plates and 

 bars ; one of which on each side bears a small tooth {^infero- 

 lateral cardiac, I), and is continued into a broad uncalcified 

 plate, lying in the hinder and lower part of the side-walls of 

 the stomach, and covered with hairs internally. There are, 

 therefore, altogether seven gastric teeth : three median, the 

 cardiac, and tlie urocardiac, and two lateral on each side, the 

 lateral cardiac and the infero-lateral cardiac. 



In the pyloric division of the stomach the food has to 

 undergo a further series of comminutions and strainings. A 

 ridge covered with long hairs projects in the median line 

 above ; other hairy ridges extend inward from the sides to 

 meet it, and nearly close the passage laterally. These ridges 

 are very convex inferiorly, and their convexities abut against 

 the concavities of an inferior median ridge, which rises up to 

 meet them, and is prolonged posteriorly into a sort of valvu- 

 lar process, covered at its termination with long hairs, which 

 bar the space left between the upper parts of the lateral 

 ridges. The concave faces of this median process are covered 



