722 A. T. MASTERMAN. 



appears, and the left collar cavity increases greatly in size. 

 Posteriorly the first trace of the left pleurocliord is seen, 

 lying laterally to the dorsal groove. In this section tlie 

 ganglion-cells are no longer seen, and the nerve is inter- 

 rupted at the pre-oral canal. 



In figs. 6 to 8 we have selected sections from a transverse 

 series. The right side of the figures is slightly posterior to 

 the left. Thus in fig. 6 the right pre-oral canal (on the left) 

 is cut throughout its length, but the left only in part. In 

 this section the pericardial sac is cut transversely, and the 

 heart is seen in its greatest size. Fig- 7 is a few sections 

 further back. Here the apex of the subueural gland is cut 

 through, and shows three internal canals and a chordoid 

 structure. Dorsally the pericardial sac is still cut, and below 

 the ventral part is the glomerulus of the ventral vessel. On 

 the right is seen the left branchial sinus leading out from the 

 dorsal sinus, and a wider right branchial sinus opposite. The 

 two horns of the ectodermal pit are also seen. Fig. 8 is still 

 further back. The pericardial sac is no longer seen dorsally, 

 but is still cut ventrally. The walls of the two collar cavities 

 are approximately in the middle line, and behind the sub- 

 neural gland will form a dorsal mesentery. The lateral nerves 

 of the post-oral ring are seen in this section. 



In fig. 9 the chief features here described are reproduced 

 in a semi-diagrammatic median section of the entire animal. 

 I have also shown the pharyngeal structure formerly described, 

 i. e. the pleurochords, the dorsal and ventral grooves, and the 

 oral grooves. 



The facts above described must inevitably tend to bring 

 Cephalodiscusinto even closer union with Balanoglossus 

 than heretofore. Not only is every organ in the central 

 complex of the former to be directly compared to its homo- 

 logue in the latter, but the latter has no organ in this region 

 which does not occur in the former. The only essential 

 difference is one which several years ago appeared to me to 

 be of fundamental importance, but which must now be regarded 

 as of secondai'y value. In Balanoglossus tlie pericardial 



