II.] THE CRAYFISH AND LOBSTER. 2lt 



readily seen to project backwards and down- 

 wards immediately in front of the intestinal 

 ccecum. 



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



intestine, 

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



orifice. 

 /?. It terminates abruptly, in a thickened border, 



immediately posterior to the ccecum. 

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

 and slightly spirally and beset by a number of 

 short papillae. 



. Examine the middle segment of the alimentary canal 

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

 lining; it bears the ccecal appendage and receives 

 the digestive ducts. 



f. 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. 



ft. 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 



143 



