THE GASTROPODA 69 



represented only by a small ventral appendage. Among the free- 

 swimming Gastropods we find that in the Heteropods the foot is 

 laterally compressed to form a vertical natatory lobe held upper- 

 most in swimming (Fig. 142), but in Phyllirhoe it no longer exists as 

 a differentiated organ (Fig. 161). In leaping Gastropods, such as 

 Hostellaria among the Strombidae (Fig. 46), the foot is also laterally 

 compressed, and its ventral surface, if not displaced anteriorly,' is 

 not flat. In Harpa the posterior part of the foot may be cast off by 

 a process of autotomy. 



The creeping sole is often divided by a median longitudinal 

 furrow ; this may be seen in sundry Rhipidoglossa, e.g. Trochus, 

 Stomatella, Phasianella ; and in Taenioglossa such as Littorina and 

 Cydostoma : in the last-named genus the two halves of the foot 

 contract alternately during progression. A transverse furrow, 

 crossing the anterior half of the foot, is found in the Olividae, 

 Pomatiopsis, many Auriculidae, Otina, and Cyerce. 



Certain parts of the foot may exhibit special differentiations. 

 (1) Its two anterior angles are prolonged into tentacles in Cyclostrema, 

 Valvata (Fig. 132), Choristes, Olirella, Eolis, etc. (2) The anterior 

 margin of the foot may be furnished with a number of small tactile 

 papillae as in Trochus, etc., or there may be a small fleshy projection, 

 called the mentum, between it and the mouth, below the aperture 

 of the supra-pedal gland, as in the Pyramidellidae (Fig. 137), 

 Siliquaria, Aclis, Vermetm. In Capulus there is a little projecting 

 tongue-shaped structure above the anterior margin of tHe foot and 

 below the snout, and in Vermdus two symmetrical tentacles are 

 present in the same position, on either side of the aperture of the 

 supra-pedal gland. (3) In various fossorial Gastropods the whole 

 anterior region of the foot is somewhat elevated above the head, 

 to form the propodium. This region is distinctly separated from 

 the rest of the' foot by a constriction in the Harpidae and by a 

 transverse furrow in the Olividae. The propodium is particularly 

 well developed in the Naticidae, in which it is reflected over the 

 whole cephalic region to form a powerful digging organ (Fig. 47). 

 (4) The lateral margins of the foot are expanded to form fins or 

 parapodia in certain Olividae, and particularly in a number of 

 Opisthobranchs, as, for example, in Gastropteron, Acera, etc. ; among 

 the Bullidae, the Pteropods, Aplysia, etc. In Xotarchus these two 

 lobes are united above the body in such a manner as to form a 

 muscular sac open in front, but closed behind and at the sides. By 

 forcibly expelling water through the anterior aperture, the animal 

 makes use of the sac as an organ of locomotion. (5) The posterior 

 region of the foot is often separated off as a distinct operculigerous 

 lobe, as may be seen in the Strombidae (Fig. 46), Xenophorus 

 (Fig. 134), and the Atlantidae (Fig. 141). In some Marginellidae 

 there is a posterior dorsal <liscoid lobe. In Nassa and in allied 



