264 Dr. R. Bergh on the 



where these arches meet together there occurs a strong thick- 

 ening (cerebral ganglion), and the inferior arch, which is the 

 further back, is tliicker than the upper one, and penetrated by 

 a strong extramedian (right) artery. It is very difficult to 

 prepare the nervous system, which appears in all parts yellow 

 or brownish yellow, out of the whitisli firmly adherent sheath, 

 which is continued to a considerable distance around the 

 thicker nerves. The ganglia are all coarsely nodular, the 

 nodules projecting strongly at the surface, sometimes pedun- 

 culate. Cerebral ganglia of rounded triangular form, some- 

 what flattened. The intercercbral commissure thin, sometimes 

 scarcely occupying one sixth of the breadth of the superior 

 ring, longer than the transverse diameter of the ganglion. 

 The left cerebro-pedal connection is very short, the right one 

 much longer. The left jyedal ganglion is larger than the 

 right one, which is submedian in position ; both flattened, of 

 oval form, giving off three or four strong nervi jjediaci] the 

 pedal commissure short*. Behind and beneath the preceding 

 ganglia lie the three visceral ganglia quite unsymmetrically. 

 The largest and rather thick right one is united almost directly 

 by a veiy short cerebro-visceral connective with the cerebral 

 ganglion, and by a viscero-pedal connective, which unites with 

 the cerebro-pedal connective, with the right pedal ganglion. 

 The riglit visceral ganglion is united by a short commissure 

 with the median (genital) ganglion, which is also situated to 

 the right, and it is also connected with the left ganglion by 

 a long and powerful commissure j behind the last-mentioned 

 commissure lies the much thinner subcerebral commissure, 

 wliich may be traced into the cerebral ganglia. The left 

 visceral ganglion is more depressed than the others, and is 

 connected with the cerebral ganglion by a tolerably long con- 

 nective, and with the pedal ganglia by a somewhat shorter 

 one. As in the Pulmonata, so also here tlie gastro-vesophageal 

 ganglia which always occur in the Doridida3 (perhaps with 

 the exception of many Polgcerafa-f) are dejicient. 



The ophthcdiiiojjhores ot the Onchidia are like those of the 

 stylommatophorous Pulmonata, and such as occur elsewdiere in 

 no Gasteropoda. If it should really be the case, as Joyeux- 

 Lafl\iie states [I.e. p. 141 [36a]), that the eye here is first of 

 all formed on the head, and only ascends afterwards with the 

 ophthalmophore, while in the Stylommatophora it is deve- 



* A douWing of this commissure, such as is clesciihed by Joyeux- 

 Lalf'uie in 0. celticum {J. c. p. 79 (303), pi. xvii. fig. 4 a, b), does uot occur 

 in this case. 



t See K. Bergh, ''Beitriige zur Kenntniss der Polyceraden III./' in 

 Verhandl. k,-k. zool.-bot. Ges. in Wien, Bd. xxxiii. (1883}, p. 167. 



