2S6 ELEMENTARY BIOLOGY. [CHAI'. 



sac, slightly subdivided into two by a longitudinal 

 fold. Four apertures will be found in its walls; 

 they are — 

 a. The oesophageal orifice, opening into its hinder 



end. 

 p. The origin of the intestine, lying below and to 



the right of a. 

 y. The apertures of the digestive ducts ; opening — 

 that of the left side just above the intestinal 

 orifice — that of the right into its apex (this duct 

 skirts the free edge of its gland). 



//. The intestine. After leaving the stomach it passes 

 upwards and forwards, altogether to the left side. 

 It then makes two turns upon itself, approaching 

 the right side as it does so, and finally courses 

 along the right border of the pulmonary sac. (Cf 

 Sect. C. § I b.) 



K. The buccal mass and odontophore. 



I. Examine the hiiceal mass from the side and make out 

 its muscles. The eonstrictor fibres have already been 

 referred to, as constituting the muscular wall of the 

 whole structure. Of those which remain the more 

 important are — 



a. The retraetor ; a large sheet, arising from its floor 

 and side walls below the sac of the radula, and 

 passing back, side by side with the immense 

 retractor pedis fibres arising from the foot, to be 

 inserted into the columella or shell axis. (Cf. 

 Sect. B. d.) 



b. 'Y\\Q. protractors ; delicate muscles arising from its 

 side walls, and passing downwards and forwards, 

 to be inserted into the cephalic integument. 



