144 THE CRAYFISH 



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 back- 

 wards respectively with forceps, when the three teeth will be 

 seen to come together sharply. 



D, The Excretory System. 



1. The kidneys, or ' green glands,' are a pair of rounded 



and somewhat flattened bodies, of a light green 

 colour, lying in the ventral part of the head, in front 

 of the mouth. 



2. The ureters are a pair of wide thin- walled sacs, lying 



one on the dorsal surface of each kidney. Each 

 ureter communicates directly with the cavity of its 

 kidney, and opens to the exterior by an aperture 

 already seen on the hinder surface of a tubercle on 

 the basal joint of the antenna. 



Inflate the ureter by blowing air in at its external 

 aperture. 



