FOOT. 11 



Generally, the anterior border of the foot is variously lobed 

 (plate 3), and these lobes are mostly of glandular structure, 

 though Keferstein has found them in Buccinum to consist of 

 interwoven meshes of muscular fibres, in which are lodged beau- 

 tiful nucleated cells. These lobes or filiform processes, like 

 those of the mantle, are tactile organs. 



When the foot is greatly expanded, as in Cymba, Harpa, 

 Doliuin, etc., it is generally laterally recurved over the shell, 

 somewhat like the mantle in Cyprsea and Marginella. In 

 Oliva (pi. 3, fig. 16) and in Ancillaria, the propodium, repre- 

 sented by triangular lobes, lies flat, while the distinctly separated 

 mesopodium covers a portion of the sides of the shell. In Natica 

 the propodium is greatl}' developed forwards, extending beyond 

 the head and reflected backwards over the latter in such manner 

 as to conceal it, with its tentacles, and the anterior part of the 

 shell itself, from view. The mesopodium in Natica, being like- 

 wise largely reflected over the shell, the respiratory cavity is 

 covered, but a canal is developed between the reflected pro- 

 podium and the mesopodium, which conveys water to the 

 branchiae. 



There is a rounded glandular opening in the sole of the foot 

 of many prosobranchiates (Pyrula, pi. 3, fig. 18), which is the 

 external opening of the pedal aqueous vascular system ; to be 

 more particularly noticed hereafter. 



Nearly all of the prosobranchiates whilst in the larval state 

 support an operculum on the dorsal side of the metapodium ; 

 with some, the operculum is fugatious, and is lost in the trans- 

 formation of the animal, yet it continues present in most of the 

 groups. Its secretion from the operculigerous mantle, and its 

 morphological relation to the shell, will be explained further on. 

 The opercular mantle sometimes extends beyond the borders of 

 the operculum itself, and is divided into processes or filaments, 

 which may, as in Ampullaria, be reflected over it. 



The foot is the organ of locomotion of univalve mollusks : 

 the anterior portion is protruded, and then by means of wave- 

 like muscular contractions of the sole, the whole mass of body 

 and shell is brought forward when the former motion is re- 

 peated. In Phasianella, Quoy and Gaimard have observed that 

 the foot is divided lengthwise by a furrow, and that when motion 



