11 



The latter class includes the byssus gland of the lamelli- 

 hranchiata. In the gastropoda the specialised glands 

 may be divided into an anterior foot gland, often opening 

 anteriorly into a transverse groove, and an unpaired 

 median gland opening into a cavity situated in the 

 middle line of the foot. It is very probable, however, 

 lhat both these are parts of the same system, and it is 

 generally believed that one or other is homologous to the 

 byssus gland of the lamellibranchs. The median 

 unpaired opening is absent in the whelk, but the anterior 

 glands are very well developed, and the pedal groove can 

 be observed quite early in the embryo. The portion of 

 the foot anterior and dorsal to the pedal groove, as well 

 as the anterior part of the ventral surface, may be used 

 as a clasping organ, and in this way the whelk can to a 

 certain extent retain its hold whilst using the proboscis 

 and radular apparatus to bore through a molluscan shell 

 or a crustacean exoskeleton. 



As stated above, the foot is highly muscular. It is 

 almost entirely composed of muscle fibres, and moreover 

 the greater part of it is one muscle — the columellar 

 muscle of the shell, which arises from the columella 

 (PI. I, fig. 3), and in average-sized specimens from the 

 inner surface of the 5th whorl, and is inserted into the 

 under surface of the operculum, and thus must pass 

 through the foot in order to reach this point. In the foot 

 it is crescentic in section, the convex side being dorsal 

 and very distinctly marked off from the narrow band of 

 more superficial tissue. The whole muscle lies here near 

 to the dorsal surface of the foot. The attachment to the 

 operculum is on the ventral side and forms an elliptical 

 area which is situated eccentrically. 



The columellar muscle of the gastropods has been 

 homologised with the adductor of the lamellibranchs by 



B 



