W. K. BROOKS ON THE GENUS SALPA. 97 



neural surface of the stolon or the oral end of the salpa, and have also 

 begun to lengthen towards the genital surface of the stolon or the 

 posterior end of the salpa, carrying with them the two perithoracic 

 vesicles, g and h, and the rudiment of the pericardium, e. The dotted line, 

 i and j, shows the inner edge of the ectodermal fold, and it also of course 

 marks the boundary of the aperture by which the body cavity of the 

 salpa communicates with those of the salpae before and behind it, while 

 the circle, d', is the boundary of the opening which connects the rudi- 

 mentary pharynx of the salpa with those of the adjacent salpae. 



The aboral ends of the endodermal pouches, 27 and 28, soon begin to 

 grow backwards, as is shown in cut 0, and they carry with them the 

 right perithoracic vesicle, </, and the left, h, and also the rudiment of the 

 pericardium, e, which, it will be seen, is in relation to the right endo- 

 dermal pouch. The oral ends of the endodermal pouches push in to 

 the middle line of the salpa, as is shown in the diagram, and also in 

 Plate V, Fig. 2, meeting each other on the ventral or distal side of the 

 ganglion, s. As these figures show the proximal or dorsal surface of the 

 salpa, the ganglion is towards the reader, and the oral ends of the endo- 

 dermal pouches are behind the ganglion. This relation is shown in 

 section in Plate XXXI, Fig. 3, B, where the ganglion is colored purple, 

 and the endodermal pouch, 27, red. 



As the endodermal pouches grow backwards (towards the bottom of 

 the stolon and towards the aboral end of the salpa), the wall of each of 

 them unites with the wall of the corresponding perithoracic vesicle, and 

 an opening or gill-slit, which puts the cavity of the vesicle into commu- 

 nication with the endodermal pouch on its own side of the body, is 

 formed at the point of union, as is shown in Plate XXVIII, Fig. 1, c, c'. 

 At C, the gill-slit which connects the right perithoracic vesicle, g, with 

 the right endodermal pharyngeal pouch, 27, is shown, and at C", the 

 one which connects the left perithoracic tube, h, with the left half, 28, of 

 the pharynx. The first trace of the gill-slit is a fold in the endoderm of 

 the dorsal or proximal wall of the pharyngeal pouch, as is shown in Plate 

 XXIV, Fig. 1, 27 and 28. 



While this change is taking place the posterior end of the right 

 pharyngeal tube gives rise to a blind diverticulum, Plate V, Fig. 3, q, 

 which is destined to become converted into the oesophagus, stomach 

 and intestine. It therefore represents all the digestive tract except the 

 pharynx, and I shall speak of it as the rudimentary gut. It lies almost 

 directly over or dorsal to the rudimentary pericardium, e, which is thus 



