154 THE CEAYFISH, 



hinges. Their lower ends are curved back- 

 wards. 



vii. The zygocardiac 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. 



viii. The filter. The apertureTsetween 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 hairlike 

 setsB, which cross one another from opposite 

 sides so as to form a very efificient 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 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 



