286 DESCRIPTION OF THE PLATES. 



insertions just anteriorly to the umbones, whence they radiate over 

 the regions of the stomach, and towards the pericardium. 



i. Left cerebro-pleural ganglion lying in the angle between the anterior 

 retractor and adductor, and the protractor pedis, above the entrance 

 to the mouth. 



j. Connective passing from the left cerebro-pleural ganglion to the left 

 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. 



f. Auditory vesicle appended to pedal ganglion. This vesicle is ordi- 

 narily said to be appended to a branch given off from the most 

 backwardly placed but one of the posterior pedal nerves. But 

 Simroth states that its nerve is derived from the cerebro-pedal 

 connective and so figures it. See Z. W. Z. xxvi. 1876, PI. XVI. 

 Fig. 56, and p. 138, ante. It is not always symmetrically developed 

 on both sides, and, when present on one side only, it has been found 

 to contain 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 its purely muscular portion into which 

 the viscera do not enter. 



k. Commissure between the left cerebro-pleural and visceral ganglia. It 

 passes between the fibres of the retractor pedis and the protractor 

 through the upper part of the foot, internally to the generative orifice, 

 / ; then through the glandular portion of the nephridium, s ; and 

 across the tendon of the retractor pedis posterior where it bifurcates 

 for insertion into either valve of the shell. 



/. Left visceral ganglion. Two nerves are figured in connection with it, 

 one, a parietal nerve, going to the mantle, the other, a branchial nerve, 

 going to the gill. 



m. Rectum ending in the cloaca. A delicate nerve is figured by Duvernoy, 

 op. cit. p. 138, ante, as passing to it from the visceral ganglion. 



n. Heart ; the letter pointing to the slit left by removal of the left 

 auricle. 



o. Pericardial space into which opens the glandular portion of the nephri- 

 dium. 



/. External opening of the nephridial duct. 



q. Opening by which the glandular portion of the nephridium communi- 

 cates with the duct. 



r. Wide opening by which the ducts of the two nephridia communicate. 

 This opening does not exist in Unto margaritifer. 



s. Secretory or glandular portion of the nephridium, reaching from the 

 level of the anterior end of the pericardial space to the under surface 

 of the posterior adductor. It opens into the pericardium by a canal 



