Mr. A. Hancock on the Anatomy of Oithona. 299 



line, across which they are united at the anterior extremity by a 

 short but distinct commissure : their posterior extremities diverge 

 and are slightly bilobed, marking the boundaries of the two 

 ganglia of which each mass is composed, — the anterior lobe indi- 

 cating the cerebroid, the posterior the branchial. In Doris bila- 

 mellata and D. aspera these two ganglions are fused, much in the 

 same manner, and have a strong general resemblance to those of 

 Oithona. The pedial ganglions (c, c) are irregularly rounded, 

 being equal in bulk to the cerebroid and branchial together. 

 They lie against the sides of the gullet, and are united to the 

 under surface of the central masses. The fourth pair of ganglions 

 are the olfactory (d, d) ; they are well developed, though very 

 much smaller than those just described, and are joined by a short 

 pedicle or commissure to the upper surface of the anterior mar- 

 gins of the cerebroid ganglions. The close approximation of the 

 olfactory ganglions to the central masses is a matter of some 

 interest. In the Dorides they are sessile upon these masses. 

 Thus we come to learn that these ganglions, which, in all the 

 Eolidida that we have examined, with the exception of this genus, 

 are placed at the base of the dorsal tentacles, and consequently 

 far removed from the central masses, are in truth cerebral gan- 

 glions, and like the olfactory of the higher animals, are placed 

 in front of the brain. 



The infra-oesophageal ganglions are placed in the usual situa- 

 tion on the buccal mass, below the gullet. The buccal ganglions 

 (e, e) are scarcely larger than the olfactory, and are of an oval 

 form, their inner extremities being connected across the median 

 line by a short commissure ; their outer extremities receive a cord 

 of communication from each of the cerebroid ganglions. Two 

 minute elliptical ganglions are almost sessile on the anterior 

 border of the buccal ganglions ; these are the gastro-cesophageal 

 ganglions {f,f). Thus in all there are six pairs of ganglions; 

 four above the gullet, and two below it. 



The first pair of nerves come from the olfactory ganglions, and 

 are large, but of no great length ; they divide into several fila- 

 ments as they enter the base of the dorsal tentacles. The second 

 pair pass from the under surface of the anterior border of the 

 cerebroid ganglions, not far from their union with the olfactory 

 ganglions ; these nerves go to supply the upper surface of the 

 channel of the mouth. The third and fourth pairs of nerves 

 issue from the same ganglions, but considerably behind the second 

 pair ; these also go to the channel of the mouth ; the third pro- 

 bably sending a branch to the oral tentacles. A strong cord 

 passes off close to the root of the fourth pair : these cords curve 

 round the oesophagus and are united to the outer extremities of 

 the buccal ganglions, forming the anterior collar (g). The fifth 



