W. K. BROOKS ON THE GENUS SALPA. 71 



fundamental identity of origin. I shall, however, discuss the subject 

 further on. 



SECTION 5. Tlte Endodermal Tube. 



This is colored red in the sections, as are also its derivatives. 



As d' in Figs. 2, 3, 4 and 5 of Plate XIX shows, it arises as a tubular 

 outgrowth from the ventral wall of the pharynx on the middle line 

 between the two folds of the endostyle, which are marked d--d in these 

 and other figures; and in Salpa pinnata and Salpa cylindrica it per- 

 manently retains its communication with the cavity of the pharynx. 

 Seeliger states that in Salpa democratica it soon loses its communica- 

 tion with the pharynx, but this species is a difficult one to study, and the 

 opening is easy to find in the straight stolons of the two species which 

 I have studied most thoroughly, even in the oldest specimens. I have not 

 found in any of my sections any food or foreign organisms in the en do- 

 dermal tube of the stolon at any stage, nor have I ever found in the 

 stomachs of the young chain-salpae any evidence that they are nourished 

 through it, although their digestive organs are derived from it, and retain 

 for a long time their communication, through it, with the pharynx of the 

 solitary salpa. It is formed, as Plate XXXV shows, before the mouth of 

 the solitary embryo communicates with the exterior, and there is no 

 reason for believing that it has a nutritive function. As the longitudinal 

 sections in Figs. 5, 6 and 7 of Plate XX show, the inner ends of its cells 

 have a ragged or indefinite outline, as have also the cells of the endostyle, 

 Fig. 2, d, and it is probably ciliated when alive, and, at later stages, when 

 the pharynx of the solitary salpa is filled with fresh sea-water, it may, 

 perhaps, convey to the young chain-salpa3 a respiratory current. 



In a mature stolon, Plate XXXIV, Fig. 1, d', its upper and lower 

 walls are formed of flattened cells, with clearly defined outlines, shown 

 more magnified in Fig. 3, while along each side of it there is a thick strip 

 of elongated cells, with their inner edges indefinite and ragged. At the 

 distal end of the tube these two strips unite, and as the longitudinal 

 section in Plate XVI, Fig. 5, shows, its tip consists of elongated cells. 



Before we examine its structure and origin in detail, it will be neces- 

 sary to briefly describe the endostyle of the solitary embryo. Plate XXI, 

 Fig. 5, shows in red at b-d-d-b a transverse section across the middle line 

 of the pharynx of the solitary embryo. The ordinary endoderm of the 

 pharynx is shown at b. It consists of flattened cells with sharp outlines. 

 The endostyle consists of two parallel ridges of greatly elongated cells 



