28o ELEMENTARY BIOLOGY. [CHAP. 



LABORATORY WORK. 



If the Snail be killed, by immersion in water heated to a 

 temperature such as the hand can comfortably bear, the 

 shell will readily part company with the muscles to which it 

 gives attachment. This being the case, no difficulty will be 

 experienced in removing the animal, if, holding it in the left 

 hand, the shell be twisted off by the finger and thumb of 

 the right. 



In dissecting the internal organs it is advisable to re- 

 move the visceral sac in starting. This may best be 

 done by cutting away its thickened edge with a pair of 

 scissors, and tearing it off with a couple of pairs of strong 

 forceps. 



A. General external characters. 



i. In the living animal observe : 



a. The body ; produced ventrally into an expanded 

 locomotor disc or foot ; and dorsally into a spirally 

 coiled visceral-sac, invested in the single coiled 

 shell or exoskeleton. 



b. The head segment ; a freely projecting anterior lobe 

 of the body, which overhangs the foot. It bears 

 two pairs of retractile tentacles; a smaller lower- 

 most labial pair, situated at the sides of the mouth, 

 and a longer dorsally placed ocular pair, at the 

 summit of each of which there is borne, when fully 

 extended, a minute black eye. 



c. The apertures. Examine the animal en face, and 

 note : 



