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 ( Fermetus, 

 Hipponyx) and in terrestrial forms such as Cydostoma 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 

 Cydostoma, in which it is composed of multiple tubules ; in Cypraeu, 

 Hemifusus (Fig. 44), Cassis, and a large number of Rachiglossa and 

 Toxiglossa, viz. in the Fasciolariidae, Turbinellidae, Nassa, Murex, 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 (Orpitlla, Plertrophorus, 

 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 of water and serves for the suspension of 

 the animal. In some species of Limax, Litiopa, Cerithidea, etc., 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 Phyllirhoe, but in no 

 Gastropod 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 



