THE GASTROPODA 71 
surface in a reversed position. (2) The supra-pedal gland opens in 
the middle line between the snout and the anterior border of the 
foot. It is most commonly found in sessile Streptoneura (Vermetus, 
Hipponyx) and in terrestrial forms such as Cyclostoma and the 
Pulmonata. It is often very deep, and extends for nearly the 
whole length of the foot: its walls are thrown into folds and are 
ciliated ventrally in the majority of the Pulmonates. (3) The 
ventral pedal pore, situated in the middle line in the anterior 
moiety of the foot, is the aperture of a more or less extensive and 
often ramified cavity into which the glands of the sole or the pedal 
glands properly so called pour their secretion (Figs. 44, B; 144, I). 
This organ is comparable with the byssogenous gland of Lamelli- 
branchs (Fig. 197), and is found in the following genera: in 
Cyclostoma, in which it is composed of multiple tubules ; in Cypraea, 
Hemifusus (Fig. 44), Cassis, and a large number of Rachiglossa and 
Toxiglossa, viz. in the Fasciolariidae, Turbinellidae, Nassa, Murea, the 
Olividae, Marginellidae, and Conidae (Fig. 144). Its opening was 
formerly mistaken for an aquiferous pore. (4) The posterior mucous 
glands may be either dorsal or ventral in position. The former are 
characteristic of terrestrial Gastropods, such as the Pulmonates and 
certain Cyclostomatidae, in which they are often surmounted by a 
simple or multiple horn-shaped protuberance (Orpiella, Plectrophorus, 
Dermatocera). The ventral posterior glands are simple localisations 
of the dermic glands, and occur in various Opisthobranchs ; they are 
not sensibly invaginated in the Pleurobranchidae and Pleuro- 
phyllidae, but are invaginated and form a long canal in Gastropteron. 
The product of secretion of the pedal glands in many cases solidifies 
on contact with the air or water and serves for the suspension of 
the animal. In some species of Limaz, Litiopa, Cerithidea, ete., it 
assumes a filamentous form ; and in both sexes of Janthina, whether 
viviparous or not, it is filled with air-bubbles and forms a float, 
covering the ventral surface of the foot, beneath which the animal 
is suspended (Fig. 135). 
The ventral border of the flattened and fin-like mesopodium of 
the Heteropoda exhibits, in the male at any rate, an invagination in 
the form of a sucker (Figs. 141, 142, d’). A similar sucker exists 
on the ventral or pedal area of some species of Phyllirhoé, but in no 
tcsiroped does the foot exhibit an aquiferous pore, in the sense 
formerly attached to this term. In some forms, however, and at 
all events in the Naticidae, there is a system of aquiferous spaces in 
the foot ; these spaces are completely separated from the circulatory 
apparatus and serve to distend the foot (Fig. 47, VIII) in the action 
of burrowing in the sand or mud. 
The foot often bears on its posterior dorsal aspect a solid 
sclerite, known as the operculum, which, on the retraction of the 
animal, serves to close the aperture of the shell. The operculum 
