66 ORGANS OF MOVEMENT IN THE GASTROPODA, ETC. 



and these lobes are mostly of glandular structure, though Kefer- 

 stein has found them in Buccinum to consist of interwoven meshes 

 of muscular fibres, in which are lodged beautiful 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, 

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

 somewhat like the mantle in Cyprjea and Marginella. In Oliva 

 (iii, 46), and in Ancillaria, the propodium, represented by trian- 

 gular lobes, lies flat, while the distinctly separated mesopodium 

 covers a portion of the sides of the shell. In Natica the propo- 

 dium is greatly 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 (Ixiii. 41). The mesopodium in Natica, being 

 likewise 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 gastropods (Pyrula, vii, 85), which is the external 

 opening of the pedal aqueous vascular system ; to be more par- 

 ticularl}^ 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 fugacious, being lost in the trans- 

 formation of the animal, j^et it continues present in most of the 

 groups. 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 

 repeated. In Phasianella, and many other genera, the foot is 

 divided lengthwise by a furrow, and when in motion the muscles 

 of the sole are alternately used on either side, so that the effect 

 of the motion is that of a pair of feet. In attached shells, like 

 Yermetus, the foot is only rudimentary and serves merely as a 

 support to the operculum. 



In retiring within the aperture of the shell the foot is generally 

 doubled upon itself across the middle, so that its dorsal posterior 

 side, bearing the operculum, comes outermost ; but in Oliva and 

 Yoluta it folds longitudinally, whilst the quadrate foot of Conus 

 is withdrawn obliquely, without folding, first the right, and then 

 the left side. 



In the Scaphopoda the foot is vermiform (iii, 43). 



