52 OPISTHOBRANCHIATA OF BRAZIL 



anterior end by the entrance of the respective connectives, and 

 by a slight groove upon the anterior face. The nerve cells of 

 these ganglia are of the usual gigantic type found in Opistho- 

 branchs generally, and cause the surface of the ganglia to present 

 a series of irregular protuberances. 



Parietal nerves. From the right or parietal ganglion arise 

 two nerves. 



1. The vulvar nerve (PI. VIII, fig. 35 ; PI. X, fig. 42, r. p. r), 

 is a delicate trunk from the right side, soon bifurcating into (a), 

 a branch (figs. 35, 42, r. p. 10) coursing forward and anastomos- 

 ing with a branch of the third pedal nerve (PI. VIII, fig. 35, J/), 

 forming the pedo-parietal connective before described, and (b), 

 the vulvar nerve proper (PL VIII, fig. 35 ; PL X, fig. 42, r. p. ib), 

 which passes to the anterior end of the large hermaphroditic duct 

 and to the integument surrounding it. 



2. The second, or osphradio-ctenidial nerve (PL VIII, fig. 35 ; 

 PL X, fig. 42, r. p. 2), is a very strong trunk, in diameter quite 

 reaching that of the pleuro-parietal connective. It arises from 

 the upper right side of the parietal ganglion, passes outward and 

 backward in a curve to the right, thence upward to the anterior 

 base of the ctenidium, where it unites with the large ganglion of 

 the osphradium (PL VIII, fig. 35; PL X, fig. 42, osp. g.}, lying 

 immediately below the integument, and visible through it. The 

 osphradium is visible externally as a depressed oval area of a 

 light yellowish color, situated upon the ventral face of the anterior 

 portion of the base of the ctenidium. It is 0.4 mm. in length by 

 o.i mm. in width. From this osphradial ganglion arise two rather 

 strong nerves, one (PL VIII, fig. 35; PL X, fig. 42, osp. g. /), 

 passing forward, its several branches being distributed among the 

 large gland cells of the anterior and lateral margins of the mantle. 

 The other nerve from the osphradial ganglion passes a short dis- 

 tance to the left and terminates in a smaller branchial ganglion 

 (PL X, fig. 42, ct. g.) at the right of the pericardium. From this 

 ganglion a main branchial nerve (PL X, fig. 42, ct. n.), passes 

 backward to the ctenidium, and several very delicate nerves are 

 also given off to the pericardial wall and are lost among its 

 fibres. 



Visceral nerves. The left, or visceral ganglion is equal in 

 size to the right parietal one. At its anterior, more pointed end it 



