146 THE CRAYFISH 



V. The median tooth is a strong calcified sclerite, at 

 the junction of the cardiac and pyloric cham- 

 bers. With its upper surface are articulated 

 the urocardiac ossicle in front and the pre- 

 pyloric behind, 

 vi. The pterocardiac ossicles are attached one to 

 each end of the cardiac ossicle by oblique 

 hinges. Their lower ends are curved back- 

 wards, 

 vii. The zygocardiao ossicles are a pair of stout 

 sclerites, running from the outer ends of the 

 pyloric ossicle forwards, downwards, and out- 

 wards, to the ventral extremities of the ptero- 

 cardiacs. Their ventral margins are curved 

 inwards to form a pair of stout plates, calcified 

 at their inner borders to form the lateral 

 teeth. The inner edge of each lateral tooth 

 bears a row of denticles, which decrease in 

 size from before backwards. 

 This gastric mill is put into action by muscles arising 

 from the carapace and attached to the cardiac 

 and pyloric ossicles, which, by their contractions, 

 they pull away from each other. The cardiac 

 ossicle moving forwards pulls the urocardiac and 

 the median tooth with it, and the prepyloric 

 ossicle rotates forwards with them, giving a down- 

 ward movement to the hinder part of the tooth, 

 and causing its apex to rotate forwards. The 

 same movement of the cardiac ossicle forwards, 

 and of the pyloric backwards, causes the zygo- 

 cardiacs to pull back the pterocardiacs, which 

 rotate about the oblique hinges, pressing the 

 zygocardiacs inwards, and bringing the lateral 

 teeth together in the median plane. 



Pull the cardiac and pyloric ossicles forwards and bach- 

 loards respectively with forceps, when the three teeth will be 

 seen to come together sharply. 



