DIGESTIVE SYSTEM. 161 



lining throughout, and its inner surface presents 

 longitudinal and slightly spiral ridges beset 

 with small papillae. 



The anus is a longitudinal slit on the ventral 

 surface of the telson : it can be closed by ap- 

 proximation of its sides. 



Remove a piece of the intestine, and examine its inner 

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

 see the ridges and papillce. 



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 antero-trans verse 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 postero -transverse sclerite 



in 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 teeth and processes are strong calcified sclerites, 

 at the junction of the cardiac and pylorio chambers. 

 These are fused with the urocardiac ossicle in front and 

 the prepyloric behind, 

 vi. The pterocardiac ossicles are attached one to each end of 

 the cardiac ossicle by oblique hinges. Their lower ends 

 are curved backwards, 

 vii. The zygocardiac ossicles are a pair of stout sclerites, run- 

 ning from the outer ends of the pyloric ossicle forwards, 

 downwards, and outwards, to the ventral extremities of 

 the pterocardiacs. Their ventral margins are curved 

 inwards to form a pair of stout plates, calcified at their 



