2 88 ELEMENTARY BIOLOGY. [CHAP. 



c. Examine the functional region under a \ or 

 objective. Note that the bases of the two sets of 

 lateral teeth (uncini), by which the radula is for 

 the most part beset, are concave externally. Follow 

 those of one side inwardly a single median longi- 

 tudinal row (rachis) will be reached, the bases of 

 whose teeth are concave on either side. 



The rasping surface of each median tooth is pro- 

 duced into a pointed cusp. In the lateral teeth there 

 appears, on the inner side of this, a second smaller 

 one which increases in size relatively, as the free 

 edge of the radula is approached. There appears, at 

 the same time, a smaller external accessory cusp, 

 which is not represented in the median teeth. 



d. The odontophoral cartilages ; two gristly masses to 

 whose presence the elevation of the floor of the 

 mouth is due. There are attached to their bases a 

 series of small intrinsic muscles, arising from the 

 side walls of the buccal mass. 



e. The horny beak ; seen to be formed in relation with 

 a special fold of the lining membrane of the roof of 

 the mouth. It lies wholly within the circular lip, 

 seen in section to be thick and fleshy. 



f. The salivary duct ; opening into the roof of the 

 mouth by a minute orifice situated just above the 

 odontophore. 



F. The Pedal Gland. 



This is best examined at this stage, by removing the foot 

 to one side of the middle line. It has the appearance of a 

 white fluffy-looking mass, lying immediately above the 

 pedal disc, and extending back for two-thirds the length of 

 the same. Open it up with care it consists of a coecal 



