982 MRS. O. A. MERRITT IIAWKES ON THE [Dec. 11, 



Vai>us 2, it sends oft' a visceral bi'ancli which has the same course 

 and disti'ibiition as the other visceral branches already desci'ibed. 

 The post-trematic divides into two parts (A and B) of almost equal 

 size, which run pai-allel to one another as in Vagus 2. The fine 

 side branches which are so numerous in the equivalent nerve of 

 Vagus 2 cannot be traced here. It is probable that branch A has 

 appi'oximately the same function as the branches which Cole calls 

 "accessory skeletal" in Chimcera (3. p. 667 &c.). 



(e) Vagus 4, 5, 6 and the truncus intestinalis are so closely united 

 that they cannot be sepai-ated for a distance of 2 cm. from the 

 vagal ganglion. There is no external trace of separate ganglia, 

 or of a separate compound ganglion for these four nerves. At a. 

 distance of 2 cm. from the vagal ganglion. Vagus 4 can be 

 dissected away. This nerve passes obliquely backward to divide 

 just above the fifth branchial cleft into a veiy fine pre-trematic 

 and a stout post-trematic. The pre-trematic could be traced only 

 for 1 5 mm., that is about 1 cm. after it gives ofi'the visceral branch. 

 The post-trematic soon divides into two equal branches which 

 run parallel. There is nothing special about the course of the 

 branches in this segment. The tendency toward a reduction of the 

 pre-trematic is cai-ried further here than in the more cephalic 

 segments. Further, it is noticeable that the pre-trematic of 

 Vagus 4 lies in close association with the two divisions of the 

 post-trematic of Vagus 3. From this association and the reduced 

 size of the pre-trematic branches, it may be deduced that the 

 pre-trematics are being replaced by the post-trematics of the 

 immediately preceding segment. Ultimately such a tendenc}^ 

 would lead to the conclition found in some Teleosts. As the pre- 

 trematic and post-trematic filaments appeal' to be equally well 

 developed, the reduction of the pre-trematic nerves can only be 

 accounted for on the supposition that some other nerves are 

 i-eplacing them functionally. 



(/') Vagus 5 can be dissected from the combined branchial aiid 

 intestinal nerve about 1*5 cm, beyond the point of sepai-ation of 

 Vagus 4. It then passes obliquely downward for about 3 cm. 

 before it divides into the pre- and post-trematics. In one case 

 Vagus 5 divided into the pre- and post-trematic near the branchio- 

 intestinal riband. The two divisions are of unequal size, the pre- 

 trematic being the smaller in two out of three nerves dissected. 

 The large size of the post-trematic is remarkable, as there are no 

 filaments upon the posterior wall of the sixth cleft, i. e. upon the 

 sixth branchial arch. 



Tl\\e j^re-trematic passes along thefifth branchial arch immediately 

 under the lining membrane, along the base of the cartilaginous 

 branchial rays. These rays separate this nerve from the two 

 parts of the post-trematic of Vagus 4. Pre-trematic 5 sends 

 small branches outwards among the branchial filaments. On one 

 side, where the forking of Vagus 5 took place at an unusual 

 distance from the cleft, a fine branch united the pre- and post- 

 trematics about 5 mm. from the fork. The visceralis is given off 



