IV.] THE COMMON SNAIL. 28 1 



a. The mouth ; surrounded by a thick circular lip^ 

 externally to which there is, on either side, a 

 well-marked lateral lip. Observe that the mouth 

 is bounded above by a denticulate horny beak^ 

 lying within the circular lip. 



fi. ThQ pedal gland ; opening by a wide aperture, at 

 the base of a depression between the head seg- 

 ment and the foot. Insert a seeker into it — it 

 can be readily introduced for a distance of more 

 than an inch. 



y. With the animal still in this position, note that 

 the shell (and visceral sac which underlies it) 

 is carried altogether to the creature's right side. 



Place the snail right side uppermost, and examine : 



S. The genital orifice, situated a short distance be- 

 hind the bases of the tentacles of the right side. 



e. Gently raise the shell, and note, underlying its 

 free edge, the thickened glandular border of the 

 pulmonary sac. Enclosed by valve-like folds of 

 this there lies the large respiratory orifice. Situ- 

 ated side by side with, and a little to the right 

 of, this, is the a?ius. (The excretory orifice also 

 opens at this point, but within the lip of the 

 respiratory one. It is not visible without dis- 

 section. Cf. Sect. C. id.) 



C The genital furrow, a feebly defined integumental 

 groove, extending from the base of the pulmonary 

 sac to the genital orifice. 



B. The shell or exoskeleton. 



a. It forms a continuous investment. Its free edge 

 is produced into a whitish porcellanous reflected 



