156 HUGH WATSON. 



triangle. But it will be seen from PI. XV, fig. 11 , and 

 PI. XVI, fig. 88, that the buccal cavity has four sides, and 

 only appears somewhat triangular owing to the narrowness of 

 the ventral wall. These figures also show the great thick- 

 ness of the sides of the buccal mass — a feature which is still 

 better seen in PL XVI, fig. 87. Externally there is a layer 

 of longitudinal muscles, which is thin in Apera dimidia 

 and A. sexangula, but somewhat thicker in A. gib- 

 bons!. Within this there is a very thick layer consisting 

 chiefly of circular muscles, but also containing some radial 

 and a few longitudinal fibres. The last become more numerous 

 towards the inner surface, so as almost to form a third 

 muscular layer in which the longitudinal predominate over 

 the circular fibres. Lastly, the cavity is lined by a compact 

 cubical epithelium which is continuous over the lips with the 

 epidermis of the skin. The cells composing this epithelium 

 are small, and their nuclei are situated towards their outer 

 ends — that is to say, the ends turned away from the cavity. 

 Towards the opening the epithelial cells become taller, 

 and the translucent cuticle which they secrete is consider- 

 ably thickened on the lips. There is, however, no jaw in 

 Apera. 



At its hind end the buccal cavity extends into a long 

 tubular diverticulum, which opens into the centre of the 

 posterior wall of the cavity. This is the radula-sac, and is 

 shown in longitudinal section in PL XVII, fig. 92, and in trans- 

 verse section in PL XVIII, figs. 116-120. It will be seen that 

 a broad fold or cushion projects like a typhlosole from the 

 upper wall into the cavity, and renders the latter crescentic 

 in section. Towards the hind end of the radula-sac the edges 

 of this cushion are spirally rolled (fig. 119). Further forward a 

 narrow longitudinal ridge occurs dorsally at each side of the 

 cushion. The epithelium on the sides and floor of the I'adula- 

 sac consists of small cubical cells, compactly arranged, and 

 not unlike those lining the buccal cavity itself ; but the epi- 

 thelium on the cushion is of a quite different character, 

 being composed of very tall and remarkably irregular cells 



