W. K. BROOKS ON THE GENUS SALPA. 101 



The diagram in cut R shows the beginning of the union between the 

 posterior divisions of the pharyngeal pouches, to form the aboral division 

 of the branchial sac. As the diagram shows, this union first takes place 

 some distance from the aboral ends of the pouches, and it gradually 

 extends both backwards and forwards. The series of sections of the 

 salpse M-M 1 and N-N', in Plates XXXII and XXXIII, show the bran- 

 chial sac at this stage. Fig. 6, M-M', of Plate XXXIII, is close to the 

 anterior border of the area of union, and while the two pouches 27 and 

 28 meet and touch on the middle line, they are not united as they are in 

 M-M' in Figs. 4 and 3, which are sections through the area of union. 

 In Fig. 1, M-M', and in Plate XXXII, Figs. 8 and 7, which is close to the 

 posterior border of the area of union, they are in contact but do not com- 

 municate, while in section G they appear as two independent tubes, the 

 sections cutting their blind aboral ends. The pericardium of salpa 

 M-M' is shown at e, in Figs. 6 and 5 of Plate XXXII. Fig. 5 cuts the 

 oesophagus at g', and Fig. 3, both the oesophagus g and the intestine h. 



Plate VI, Fig. 4, is a proximal or dorsal view of two salpae at this 

 stage, the left-hand one being in the foreground, and the right-hand one 

 in the distance. Fig. 2 of Plate VII is a distal or ventral view of the 

 left-hand salpa of Fig. 4. It shows the pharynx as a continuous 

 chamber, the two pouches of Fig. 1 having completely united with each 

 other on the morphological middle line. 



The next diagram, cut S, shows the young chain-sal pa with all its 

 important organs fully formed, seen in dorsal view, as it would appear if 

 there were no secondary changes. It is shown at the same stage in side 

 view in the diagram, cut M, III. Its general anatomy and its relations 

 to the adjacent salpse are so clearly shown by the two diagrams that 

 explanation scarcely seems to be needed, although I give diagrams of 

 three sections across its body. In these sections the dorsal or proximal 

 surface of each salpa is at the top of the figure, and the right half on 

 the right side. Section T passes through the ganglion L, and below 

 this through the oral region of the pharynx 27, 28. Section U passes 

 through the region where the halves of the pharynx open into the endo- 

 dermal tube D, and communicate through this with the pharyngeal 

 chambers of adjacent salpae. 



In the plane of this section the ectoderm, a, of adjacent salpse is con- 

 tinuous, as well as the endoderm 6, and their body cavities also com- 

 municate with each other in the circular space between the ectoderm and 

 the endoderm. Section V passes through the cloaca; the gill, o; the 



