THE CRAYFISH AND LOBStER. 2ll 



readily seen to project backwards and down- 

 wards immediately in front of the intestinal 

 coecum. 



d. Examine, in like manner, the cuticular lining of the 

 intestine. 



a. It is continuous with the exoskeleton, at the anal 

 orifice. 



p. It terminates abruptly, in a thickened border, 

 immediately posterior to the coecum. 



y. Examine it under a lens ; it is folded longitudinally 

 and slightly spirally and beset by a number of 

 short papillae. 



e. Examine the middle segment of the alimentary canal 

 (head of the intestine) : it is destitute of chitinous 

 lining; it bears the coecal appendage and receives 

 the digestive ducts. 



/ Examine the interior of the stomach and note the 

 characters and relations of the gastric teeth. 



a. The median tooth ; a triangular red-brown struc- 

 ture, set on at the point of apposition of the two 

 median ossicles. 



It will probably be seen in section. 



y8. The lateral tooth ; larger but similar in character, 

 and situated (when at rest) in front of and below 

 a. It is carried by the postero-lateral ossicle. 



y. The accessory lateral tooth; a small denticle, borne 

 upon the posterior end of the accessory lateral 

 ossicle (§ 3. c). 



g. The lateral cardio-pyloric valve; a thickening of 

 the side wall of the stomach, immediately below 



14 — 2 



