W. K. BROOKS ON THE GENUS SALPA. 



29 



lial capsule. This pit is the perithoracic tube or spiracular tube, and its 

 external opening, which soon closes, is the spiracle. Before the two peri- 

 thoracic tubes lose their external openings they elongate, and pushing 

 across the body cavity into the substance of the visceral layer of follicle 

 cells, they meet and unite on the middle line to form the cloaca. 



A reconstruction in a transverse vertical plane, from the 

 horizontal section shown in Plate XVII, Fig. 5, and in 

 Plate XII. 



This stage in the history of the perithoracic tubes is shown in Plate 

 XII, which is a series of horizontal sections of a young specimen of Salpa 

 pinnata, and in cut B, which is a vertical transverse section constructed 

 from the series of horizontal sections. 



The reader who wishes to understand the structure of the embryo 

 must compare these figures with each other. The double fold of the 

 embryo sac, 21 and 22, and the epithelial capsule, b' or B', appear in all 

 the sections, but as they have already been described, it only remains to 

 point out that while these membranes are shown in the cut separated 

 from the embryo by an empty space, as they are in the living embryo 

 and in unshrunken specimens, they are represented in the horizontal 

 sections as close to the surface of the embryo, as they are in specimens 

 which have been imbedded in melted paraffine, which causes these deli- 

 cate unsupported folds to shrink. 



The plane of section 1, Plate XII, cuts the right spiracle g" close to 

 its external opening, while it cuts the left one below its opening and just 

 above the point where it communicates with the cloaca which is shown 

 at g'" in section 2. 



The epithelium which lines the cloaca and perithoracic tubes is 

 derived from the somatic layer of follicle cells, but in the plates it is 

 colored orange like the blastomeres, as I did not obtain proof of its folli- 



