22 GILBERT C. BOURNE. 



A few details may be added relative to the structures 

 connected witli the buccal cavity and oesophagus. 



In the mid-dorsal line the root" of tlie buccal cavity is 

 dee])ly folded to form a median ridge containing a narrow 

 lumen T-shaped in transverse section. This lumen of course 

 communicates freely below with the buccal cavity. This 

 median fold or ridge is dee[)e.st anteriorly over the mouth, 

 and extends backwards for about two thirds of the length 

 of the buccal bulb, gradually shallowing posteriorly till it 

 dies out altogether. The walls of this ridge are composed of 

 simple columnar cells, the internal ends of which iiave a 

 striated border, and bear short cilia. On either side of the 

 anterior part of this mid-dorsal ciliated groove is a somewhat 

 shallower but still conspicuous groove appearing on the 

 dorsal surface as a pair of folds lying close and parallel to 

 the median ridge. These may be called the salivary grooves, 

 for the small, simple, tubular buccal or anterior salivary 

 glands open into them near their anterior extremities (fig. 21, 

 sg. and s. d.). These anterior salivary glands are simple 

 short cieca lined by an epithelium, consisting mainly of large 

 finely granular secretory cells with a few columnar supporting 

 cells between them. The salivary grooves die out posteriorly 

 at the point where the oesophagus leaves the buccal cavity, 

 and at this level a second or posterior pair of salivary glands 

 opens into the roof of the buccal cavity, just to the outside 

 of the salivary grooves. These posterior salivary glands are 

 very small tubular structures with minute lateral diverticula. 

 They correspond in position to, but are much smaller than, 

 and not so much branched as the second pair of salivary 

 glands in Fissurella. Otherwise the struetures just described 

 are identical in the two genera. As soon as the oesophagus 

 is separated from the buccal cavity its right and left walls 

 are produced into the broad and flattened oesophageal pouches, 

 but from the first the right-hand pouch is considerably larger 

 than the left. The T-shaped lumen of the dorsal ciliated 

 groove may be traced for some way along the roof of the 

 oesophagus, but presently it dies out, and is replaced by a 



