W. K. BROOKS ON THE GENUS SALPA. 119 



the upper part of the axial tube becomes thin and flat and loses all traces 

 of segmentation. 



Section H-H' of Plate XXX, Fig. 1, cuts the left connecting tube, H', 

 at its opening into the endodermal tube. The section also cuts the lower 

 wall, d", of the endodermal tube, crosses the upper part of the lower 

 blood space, i, and cuts the right connecting tube, H, 27, just below its 

 opening into the endodermal tube. 



Section H of Plate XXIX, Fig. 2, cuts the oral end of the pharynx 

 and of the ganglion, s, but the left half of the section is not shown in the 

 figure, although F' shows what it would be if it were present. In 

 Plate XXVIII, H, sections 1 and 2 pass through the ganglion, s, and the 

 oral end of the pharynx, 27, and the sections in Plate XXVII, H, cut the 

 right pharyngeal tube below the ganglion. 



The section H-H', in Plate XXVI, Fig. 2, cuts the left connecting 

 tube at H 1 just before it begins to expand in the body of the salpa, and it 

 also passes through the lower blood space, i, close to its floor. In Fig. 1, 

 H-H', the right pharyngeal pouch, 27, and the right perithoracic tube, g 1 . 

 are cut through the aperture or gill-slit which unites these cavities. 

 The left pharyngeal tube, 28, is much flattened dorso-ventrally, as is also 

 the left perithoracic vesicle, and this latter is much elongated towards 

 the middle line of the dorsal surface. In Plate VI, Fig. 1, this elongated 

 portion of the left perithoracic vesicle is marked tf". It is the cloacal 

 diverticulum shown at G'" in cut O. In the diagram the cloacal out- 

 growths from the two perithoracic vesicles are represented as equal, 

 but in Salpa pinnata the one which is derived, in the right-hand salpa, 

 from the left vesicle, Plate XXVI, Fig. 1, H', h, and in the left-hand 

 salpa, Fig. 1, G, from the right vesicle, g, appears much larger than the 

 other in sections. This section also shows on the dorsal surface, between 

 the two perithoracic vesicles, the two folds of the follicle which are to 

 give rise to the fertilizing duct of the egg, and, nearer to the ventral 

 surface, between the pharyngeal pouches, the single fold which is to 

 give rise to the testis. 



Plate XXV, Fig. 6, H-H', shows the egg, m, in its follicle on the 

 middle line of the dorsal surface between the perithoracic tubes g and h, 

 and Fig. 4, H-H', shows on the left the blind end of the left pharyngeal 

 tube and the aboral surface of the left perithoracic tube. On the right 

 the section passes through the digestive tract, g 1 , and the pericardium, e, 

 which is on the ventral surface of the right end of the body, and so 

 directly behind the digestive tract that it is completely hidden behind it 

 in a dorsal view like Plate VI, Fig. 1. 



