W. K. BROOKS ON THE GENUS SALPA. 99 



Plate XXVI, Fig. 3, B-B', is close to the aboral end of the diagram, 

 and it cuts the two testicular folds, m, colored blue, and also the egg, n, 

 colored orange, in its follicle. 



Cut P is a diagram of a stage a little older, in which the oral ends of 

 the pharyngeal pouches have united with each other on the morpho- 

 logical middle line of the body, ventral to the ganglion, L, to form the 

 oral end of the pharynx. This union first takes place at the extreme 

 tips of the pouches, but it runs back rapidly and soon reaches to the 

 endodermal tube of the stolon, d'. Plate XXX, Fig. 3, E', 28, shows the 

 two pouches with their walls fused, but with no connection between their 

 cavities. 



At the aboral end of the body the two pharyngeal pouches incline 

 towards each other, and each perithoracic tube has given rise to an out- 

 growth which extends inwards towards the middle line of the dorsal 

 surface, where it is ultimately to meet and fuse with its fellow from the 

 opposite side to form the atrium. The salpa, which is marked H-H', in 

 Plate XXV, Figs. 5, 6 and 7, and Plate XXVI, Fig. 1, shows the elongation 

 of the perithoracic vesicles, g and h, towards the middle line of the dorsal 

 surface, and this is also shown at g'" in Plate VI, Fig. 1. These various 

 figures also show that the position of the perithoracic vesicles is such, that 

 as they grow towards the middle line they cover up the egg, so as to hide 

 it from direct observation in a dorsal or proximal view. The blind end of 

 the rudimentary digestive tract is now dilated to form the stomach, p', 

 which is joined by a short oesophagus, q, to the posterior end of the right 

 pharyngeal pouch. 



In the next diagram, cut Q, the oral ends of the pharyngeal pouches 

 have united with each other completely, so that a cross section of this 

 region would now show only a single chamber, and the posterior ends of 

 the aboral pouches are beginning to approach each other, although they 

 have not yet united. The perithoracic pouches have, however, met and 

 united on the middle line of the dorsal surface to form the cloaca, 

 fli'", which is also shown at g'" in Plate VI, Fig. 2. Plate XXXIII, Fig. 

 3, K-K, shows the perithoracic pouches, colored green but not lettered 

 in this figure, at the stage when they have met on the dorsal (proximal) 

 middle line, but before their cavities have united. It will also be seen 

 that while the posterior pharyngeal pouches, 27 and 28, K-K', Plate 

 XXXIII, Figs. 6, 5, 4, 3, 2, 1, and Plate XXXII, Figs. 8, 7 and 6, have 

 nearly met on the middle line, they have not yet united. The digestive 

 tract is now divided into an oesophagus, q; stomach, p', and intestine, 



