The nudibranch family Dironidae. 529 
latter and bifurcates, the internal branch passing to the retractor 
muscle of the bulb, the outer branch being distributed to the muscles 
of the mouth region. 
c5, the slender optic nerve, arises from a small spherical gan- 
elion, the optie ganglion, upon the postero-lateral margin of the 
cerebral ganglion. The nerve is closely attached to the anterior 
margin of c6 (or pl. 2) a nerve which arises close behind and below 
the optic ganglion, and which it accompanies out nearly to the 
eyes, which are located immediately behind the rhinophores, below 
the integument. 
c6 (or pl. I) arises close to the pleuro-pedal connective in the 
angle formed by the union of the cerebral and pleural ganglia. 
Without careful reconstruction of serial sections it is impossible to 
determine its exact origin. DREYER (1910) identifies a similar nerve 
as arising from the pleuro-pedal connective and terms it a pleural 
nerve in several Aeolids, and I have described (1909) and figured 
for Sparilla braziliana a nerve of similar distribution and of un- 
doubted pleural origin so that this nerve may be safely regarded as 
pleural likewise. It branches to the dorsal integument in the neigh- 
borhood of the rhinophores, and sends a branch to the retractor 
muscle of the pharyngeal bulb, the main branches however passing 
to the cerata of the head region and to the integument at their bases. 
The otocyst rests upon the surface of the pedal ganglion 
in contact with the side of the cerebro-pleural ganglion, immediately 
behind the optie ganglion. 
pl. 2, the second pleural nerve arises from the upper surface of 
the ganglion and breaks up into four main branches. The inner- 
most of these passes backward near the median line to the integument 
of the pericardial region. The longest branch courses backward 
along the upper margin of the side, at the bases of the cerata, and 
gives off branches to these organs. The remaining two branches pass 
to the integument of the head region and also to the anterior cerata. 
In Dirona albolineata a slender branch of one of these subdivisions 
of pl. 2 was traced forward to an anastomosis with a branch of the 
slender twig from c7, the rhinophore nerve, an arrangement similar 
to that termed by Dreyer (1910) the “outer commissure”. Such 
peripheral anastomoses are by no means of rare occeurrence in 
Opisthobranchs. 
c-b.c. From the ventro-lateral margin of the cerebral ganglia 
the short cerebro-buccal connectives arise and curve around under 
Zool. Jahrb., Suppl. XV (Festschrift für J. W. Spengel Bd. ]). 34 
