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 Phyllirhoé it no longer exists as 
a differentiated organ (Fig. 161). In leaping Gastropods, such as 
Rostellaria 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. Tvochus, 
Stomatella, Phasianella; and in Taenioglossa such as Littorina and 
Cyclostoma: 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, Olina, 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, Olivella, Eolis, etc. (2) The anterior 
margin of the foot may be furnished with a number of small tactile 
papillae as in 7vochus, 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, Vermetus. In Capulus there is a little projecting 
tongue-shaped structure above the anterior margin of the foot and 
below the snout, and in Vermetus 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, ete. In Notarchus 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 discoid lobe. In Nassa and in allied 
