194 Description of the Plates. 



(J. Protractor of the foot. This fan-shaped muscle spreads over 

 the external surface of the foot^ from an insertion into the 

 shell, a little superiorly to the point where the pallial line 

 joins the impression for the anterior adductor. It must 

 act consequently, as an antagonist to the preceding and suc- 

 ceeding muscles. Its impression is distinct in this animal 

 from that of the adductor. 



h. Anterior retractor of the foot. The fibres of this muscle take 

 origin from a point in the shell, towards the dorsal aspect 

 of the anterior adductor, though some way from its dorsal 

 border. They spread thence into the foot especially along 

 its anterior edge, and down as far as its anterior angle, oc- 

 cupying for the most part a deeper level than the preceding 

 muscle. Some of its fibres, however, spread superficially 

 over the liver region dorsally. Its action is that of a 

 powerful retractor of the foot mass. 



//. Smaller retractor muscles with insertions just anteriorly to 

 the umbones, whence they radiate over the regions of the 

 stomach, and towards the pericardium. 



i. Labial ganglion lying upon the anterior retractor, and in the 

 angle between that muscle, the anterior adductor, and the 

 protractor pedis, above the entrance to the mouth. 



j. Cord of commissure passing from labial ganglion to pedal. 

 The pedal ganglion of each side gives off twelve nerves, six 

 from its neural, and six, more slender, from its lateral surface. 

 They are not figured in this plate. 



/. Auditory vesicle appended to pedal ganglion. This vesicle 

 is ordinarily found to be appended to a branch given off 

 from the most backwardly-placed but one of the posterior 

 branches given o'S. from the pedal ganglion. It is not 

 always to be found symmetrically developed on both sides, 

 and, when found on one side only, it has been found to con- 

 tain two otoliths. It is situated in a part of the foot narrow 

 from side to side, at the junction of its anterior two-thirds 

 to its posterior third, and near to the purely muscular 

 portion of the foot, into which the viscera do not enter. 

 Cf. Moquin Tandon, Hist. Melius, i., p. 136; Duvernoy, 

 Memoires de Flnstitut, tom. xxiv., p. 96. 

 h. Cord of commissure between labial and parieto-splanchnic 



