172 HUGH WATSON. 



l^]nglandina of\he flexor muscles in Apei'a and Natalina. 

 It is interesting to notice that while the odontophore in the 

 Oleacinid^ is usually curved as in the RhytididjB, in 

 several features of its internal structure it appears to resemble 

 the odontophore of Testacella more than does that of 

 A p e r a . 



The Buccal Reteactors and Protractors. — In all the 

 species of Apera excepting A. gibbon si and A. parva, the 

 extrinsic buccal retractor consists of a long, narrow muscle, 

 arising from the right side of the floor of the body-cavity at 

 the entrance to its funnel-shaped prolongation beneath the 

 mantle-cavity or lung. The origin of the buccal retractor is 

 thus nearly as far back as the heart, and is slightly asym- 

 metrical (PI. XV, fig. 75). In front the muscle bifurcates just 

 before reaching the odontophore, and the two branches are 

 inserted one on each side of its posterior extremity (PI. XXI, 

 figs. 132-134). As might have been expected, the retractor 

 is narrower in A. burn up i and A. sexangula than in A. 

 dimidia with its larger odontophore. 



In Apera gibbonsi and A. parva the retractor is split 

 up into a number of powerful muscles, which radiate from 

 the hind end of the odontophore, and are attached to a 

 considerable area of the floor and the lower part of the sides 

 of the body-cavity (PI. XXI, figs. 129-131). In A. 

 gibbonsi these muscles originate towards the hind end of 

 the body-cavity, though further forward than the origin of 

 the single buccal retractor in the other species (PI. IX, 

 fig. 27), but in A. parva the muscles have become shorter 

 and arise from about the middle of the cavity (fig. 28). 

 These muscles are often forked and neighbouring strands 

 occasionally anastomose, but there is frequently a slight 

 gap dividing those on the right side from those on the left. 

 It is easy to see that the same carnivorous habits which have 

 led to the enlargement of the radula and odontophore in A . 

 gibbonsi and A. parva would produce a corresponding 

 development and multiplication of the buccal retractors ; and 

 it is also evident that it would be an advantage if the ends of 



