344 THE DOLABELLINAE. 



Parietal nerves.— The first parietal, or vulvar nerve (Plate 3, fig. 2, p. 1) 

 arises from the ventral surface at the left or posterior end of the ganglion. It 

 emerges from the capsule between the two ganglionic components of the parieto- 

 visceral group, but sUghtly nearer the parietal one, into which it can be traced. 

 It passes backward in the connective-tissue attached to the aorta (Plate 4, 

 fig. 1, p. 1) to the dorsal body-wall, and thence directly to the anterior end and 

 external opening of the large hermaphroditic duct. It gives off a branch later- 

 ally, which curves to the right and forward, uniting with the branch 7a from the 

 seventh pedal nerve, which is distributed to the organ of Bohadsch, which rela- 

 tion is the same as that found in Tethys cervina (Plate 3, fig. 1 ) , the pedal nerve 

 in that instance being the third in serial order. 



From the posterior end of the right parietal ganglion arises a single strong 

 nerve, the second parietal, or osphradio-ctenidial nerve. Its general relations 

 are shown in Plate 3, fig. 2, p2, and the detail of its ramification in Plate 4, 

 fig. 1, p2. It curves backward along the left border of the aorta, crosses above 

 it, and opposite the middle of the V-shaped heart penetrates the dorsal peri- 

 toneum, giving off the following branches : — Plate 4, fig. 1 , i , a slender nerve to 

 the peritoneum, 2, a nerve which passes directly forward near the median line 

 below the spermatic furrow, to which it ramifies, also sending off the branch 2a, 

 which fuses with a branch of the 7th pedal nerve, 7p. The main trunk is 

 markedly thickened near the origin of these branches, and bifurcates into the 

 nearly equal divisions 3 and 4. The ramus 3 forks into 5 and 6, the first pass- 

 ing to the anterior portion of the mantle, the second to the outer face of the 

 kidney, giving off the branch 7 to the ventral face of the same organ. The 

 ramus 4 thickens decidedly, though no well-marked ganghon is formed, and 

 gives off the branches 8 and 9 to the walls of the venous sinus at the base of the 

 heart and to the mantle, and also numerous deUcate short twigs which pass 

 through the integument to the osphradium, at the anterior end of the ctenidium, 

 indicated in the figure cited by the outhne osp. Beyond the thickened portion 

 the main nerve gives off the branch 10, which ramifies to the roof of the branchial 

 chamber above the ctenidium, and to the walls of the branchial sinus at its base. 

 The main nerve-trunk 11, now enters the ctenichum and continues as its chief 

 ctenidial nerve, cl. n, along the greater curvature to its tip, giving off numer- 

 ous lateral branches to its different divisions. Other than the smoothly out- 

 Uned thickening indicated above there is no trace of a distinct osphradial or 

 ctenidial ganghon. 



Respiratory, Circulatory, and Excretory Systems.— The external 



