162 THE CRAYFISH. 



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. 



viii. The filter. The aperture between the cardiac and pyloric 

 chambers is much narrowed by lateral folds ; and the 

 cavity of the pyloric chamber itself is reduced by two 

 lateral and a median ventral fold to a narrow three-rayed 

 fissure. 



All these folds bear short close-set hair-like setae, which 

 cross each other from opposite sides so as to form a very 

 efficient filter or strainer, permitting the passage only of 

 fluid or of very finely divided solid particles. 



The 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 downward 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 backwards respectively 

 with forceps, when the three teeth will be seen to come together. 



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 segment of the antenna. 



Inflate the ureter by blowing air in at its external aper- 

 ture. 



