MORPHOLOGY OF THE HBLICINID^. 795 



near the left anterior edge of the operculum, between it and the 

 membranous flap that surrounds the opercular lobe. This tube is 

 lined throughout by a glandular epithelium composed of rather 

 tall goblet-cells with deeply staining basal nuclei and clear cell- 

 contents. 



If, now, we enquire into the morphological significance of this 

 peculiar organ, I think there can be little doubt as to its homology. 

 It occupies the same position and receives the same nei've- supply 

 as the crypt into which fits the curved process of the opercuhun in 

 the Neritid.ie. This process is no longer- to be seen on the 

 operculum of the Helicinidse, but from a study of the muscular 

 attachments in the two groups I am inclined to think that the 

 cartilaginous plate described above represents its inner extremity, 

 all direct connection with the operculum being lost. In 

 connection with it new structures have been formed, viz. the sac 

 and the glandular tube. What its physiological significance is it 

 is hard to say. It is clear that, by contraction of the muscular 

 bands attached to it, the sac may be widely dilated, and when 

 dilated, air must flow into it through the glandular tube. The 

 abundant nerve-supply and the character of the columnar epithe- 

 lium bearing short stiff cilia suggest that the sac has a sensory 

 function, and it is possible that it may be a special sense-organ, 

 likely enough of an olfactory character, which enables the animal 

 to receive impressions from the external world when retracted 

 into its shell. For the opening of the glandular tube is in such 

 a position that it would open to the outer air whenever the 

 opercular plug was ever so little loosened. 



The pedal gland, an organ which is absent in the aquatic 

 Neritidse, is largely developed in the Helicinidse. Its position and 

 general structure are roughly indicated in fig. 45 (PI. XXXVIII.). 

 It consists of a main duct below the pleuro-pedal nerve-mass and 

 extending some little way but not far back below and between the 

 pedal cords. The duct is lined by a columnar ciliated epithelium 

 raised on the ventral side into two prominent ridges, one on either 

 side of the middle line. Suirounding the duct are bunches of 

 unicellular glands, which penetrate among the interlacing muscle- 

 fibres of this region of the foot. Each unicellular gland is 

 prolonged into a fine duct which passes between the epithelial 

 cells of the main duct and opens into its lumen. The histology 

 of this gland is reminiscent of that of the byssus gland of 

 lamellibranchiate molluscs, described by me in another place. 

 The main duct opens at the anterior end of the foot, in the mid 

 line, below the snout. 



Mention may be made here of the large mucous gland which I 

 agree with Thiele in identifying as the hypobranchial gland. It is 

 of relatively enormous size in the Helicinida?, but in structuie and 

 position does not differ much fi'om what I have described for the 

 Neritidas. It opens into the mantle-cavity (PI. XXXY. figs. 28 

 & 30) just in front of the aperture of the vaginal duct and, as shown 

 in figs. 30 to 35, it forms a considerable glandular mass bulging- 



