PATELLA. 215 



yellowish epithelium and contains mucus-secreting cells 

 in its roof. 



(/) The floor of the buccal cavity is muscular with a 

 slight sub-epithelial median development of chitin. 



(g) The odontophore and its muscles are described later on. 



The Gullet (figs. 6, 10, 15, 16, 22) runs back as a thin- 

 walled tube from its point of junction with the buccal 

 mass, overlying the central part of the posterior two- 

 thirds of this. Immediately behind the buccal mass it 

 shifts slightly to the left of the median line and merges 

 into the thick-walled crop, dilating into two ventro- 

 lateral oesophageal pouches just before the point of 

 junction (fig. 10). The oesophageal pouches are in part 

 spare tissue for the purpose of avoiding strain when the 

 buccal mass is extruded for feeding. They are probably 

 homologous with the lateral oesophageal pouches of 

 Pleurotomaria and Haliotis, though their functions cannot 

 be quite the same, as in these two forms they are lined by 

 ridged glandular epithelium, while in Patella they me 

 comparatively smooth, and not markedly glandular. The 

 gullet has various folds (figs. 15 and 22) as follows: — 



(a) A transverse section taken near the posterior end 

 of the buccal mass, shows the gullet as a dorso-ventrally 

 flattened structure, the central portion of which is marked 

 off from the sides by two long folds projecting downwards 

 from the dorsal wall. 



(b) There are a pair of longitudinal ventral folds 

 starting slightly behind the junction of the gullet and 

 the post-palatal section of the buccal cavity. These folds 

 converge as they run back, and the triangular space 

 between their front ends is marked by slight transverse 

 ribs. It seems to correspond with the much better 

 developed " ventral valve " of this region in most primi- 

 tive Prosobranchs. 



