Thompson, Brains of Cerehratiihis. 643 



missures between the oesophageal nerves; the first of these is 

 slender and thin, the second is also slender, but the third is thick 

 and surrounded by ganglion cells. On p. 320, Burger states 

 "An der namlichen Stelle" (Abgangstelle der Schlundnerven) 

 "befindet sich die erste Durchbrechung der senkrechten Querwand 

 der Gehirnkapsel, durch welche ein Faseraustausch zwischen den 

 Faserkernen der beiden ventralen Ganglien stattfindet." This 

 "Faseraustausch" is figured on taf. 25, fig. 5, and evidently repre- 

 sents a delicate additional commissure, although Burger does 

 not describe it as such. 



Montgomery (1897) describes in the brain of Cerebratulus 

 lacteus a second commissure between the ventral brain lobes, 

 posterior to the broad first ventral commissure, and of much 

 smaller size, and also three between the oesophageal nerves, mak- 

 ing a total of five ventral commissures. In Lineus sp. Mont- 

 gomery finds, posterior to the great first ventral commissure, a 

 second and a third slender commissure between the ventral lobes, 

 and four between the oesophageal nerves, giving a total of seven 

 ventral commissures for this closely related genus. 



CoE (1895, pi. X, fig. 8) figures the dorsal, the first ventral, and 

 a single oesophageal commissure in the brain of Cerebratulus 

 lacteus. 



h. The conunissures of Cerebratulus lacteus. — My observations 

 agree with those of the above mentioned workers in the general 

 features of the brain anatomy, but difi'er from them in the number 

 and the character of the ventral commissures, which must be 

 taken to include those between the ventral brain lobes, whether 

 originating in the cellular sheath or in the fibrous core, and those 

 between the oesophageal nerves. 



Fig. I is a reconstruction from camera drawings of successive 

 sections, and represents the brain as seen in horizontal optical 

 section. The dorsal brain lobes, D L, are formed by the union of 

 numerous small branches which originate in the tip of the head, 

 on each side of the rhynchodeum. The arched dorsal commissure 

 D C, unites the two dorsal lobes, and gives off' from its median 

 anterior surface a delicate nerve that runs forward to the tip of the 

 head. A similar delicate nerve, the median dorsal nerve, tn d n, 

 arises on the median posterior surface of the dorsal commissure 

 and runs backward. Just posterior to the dorsal commissure 

 the ventral brain lobes, V L, are diff^erentiated trom the dorsal 



