DIGESTIVE SYSTEM. 153 



surface under water with a low power of the microscope, to 

 see the ridges and papilla. 



d. The gastric mill is a complex structure made 

 up of numerous sclerites, or hardened portions 

 of the cuticle lining the stomach, by means of 

 which the chewing of the food is effected : the 

 principal sclerites will alone be described here, 

 viz. five median dorsal pieces and two pairs of 

 lateral pieces, the others being small and of less 

 importance. 



Remove the entire stomach from the body, avoiding injury 

 to the structures around the oesophagus ; place it in a dish 

 under water, and carefully clean away the muscles. 



i. The cardiac ossicle is a broad transverse sclerite 

 in the roof of the cardiac chamber of the 

 stomach. 



ii. The urocardiac ossicle is a median dorsal sclerite 

 in the hinder wall of the cardiac chamber. 

 Its upper end is broad, and hinged to the 

 hinder edge of the cardiac ossicle ; its lower 

 end is narrow, and directed downwards and 

 backwards. 



iii. The pyloric ossicle is a narrow transverse 

 sclerite in the anterior part of the dorsal wall 

 of the pyloric chamber. 



iv. The prepyloric ossicle lies in the anterior wall 

 of the pyloric chamber, and is hinged above 

 to the anterior border of the pyloric ossicle. 



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 



