THE CRAYFISH 



127 



urocardiac rod lies nearly in a straight line with the cardiac 

 ossicle. At the same time the lateral pieces of the frame, the 

 pterocardiacs and zygocardiacs are bowed outwards and 

 recede from one another. When, on the contrary, the 

 cardiac and pyloric ossicles are pulled apart from one 

 another, the prepyloric ossicle is moved vertically down- 

 wards and forwards, and the lateral pieces of the frame are 

 straightened out and approximated, causing the teeth on the 



car. 



Fig. 29 



The gastric mill of the crayfish. The zygocardiac have been disarticulated 

 from the pterocardiac ossicles, and the cardiac ossicle has been pulled 

 forward so as to straighten out the angle formed by the urocardiac 

 and prepyloric ossicles, car, cardiac ossicle ; pt.c, pterocardiac ossicles ; 

 ur.c, urocardiac ossicle ; p.py, prepyloric ossicle ; t, its median tooth ; 

 py, pyloric ossicle ; zy.c, zygocardiac ossicles ; It, their lateral teeth. 



inner sides of the zygocardiacs to come together and clash 

 against the median teeth of the prepyloric and urocardiac 

 ossicles. The tension of the walls of the proventriculus tends 

 to keep the cardiac and pyloric ossicles approximated, but 

 they can be pulled apart by powerful muscles, one pair of 



