APLYSIA. 209 



3. A region of irregular folds which are more complex 

 dorsalfy where they form a channel which is gradually cut off 

 from the main cavity by two lateral inwardly projecting folds, 

 thus forming the pharyngeal canal. (Fig. 22, ph. can.) The 

 edges of the pharyngeal folds bear a few scattered " stick 

 cells," differing slightly in shape from those of the jaws. 

 (Fig. 22.) This region is abundantly supplied with mucus 

 from the salivary glands which discharge into the cavity of the 

 buccal mass. (Fig. 22, r. sal. d.) 



4l. The odontophore. This arises as a cushion-like 

 projection from the floor of the posterior cul-de-sac of the 

 buccal mass, and has the shape of a pulley fixed on its postero- 

 ventral side. Over the surface of the pulley lies the radula, 

 whose posterior immature end is lodged within a pouch known 

 as the radular sac. 



The cushion or rotella is a highly muscular organ supported 

 by paired " cartilages " of a spongy nature and covered with 

 the epithelium of the buccal cavity. It is held in place by 

 muscle bands, which attach it to the wall of the buccal mass 

 and allow the whole apparatus to be thrust forward or with- 

 drawn. The cartilages also can be moved one on the other so 

 as to give a lateral movement, but this is only possible in a 

 very small degree. 



Over the posterior, dorsal, and anterior faces of the rotella 

 is stretched the radula or rasping organ. (Fig. 9, rod.) This 

 is a broad ribbon consisting of a basal or sub-radular membrane 

 in which are set numerous chitinous teeth arranged in close 

 and regularly placed transverse rows. A delicate protective 

 layer of connective tissue covers the posterior part of the ribbon. 

 The radular teeth are minute denticles whose cusps are directed 

 backwards. The central tooth of each row differs from the 

 lateral teeth, which are all alike. (Fig. 10.) In Aplysia 

 punctata there are fifteen lateral teeth on each side so that 

 the radular formula is 15.1.15. The number of laterals, 



