W. K. BROOKS ON THE GENUS SALPA. 83 



pharyngeal pouch and opens into it, as is shown in section in Plate 

 XXVI, Fig. 3, E-E', where the perithoracic vesicles, g and h, colored 

 green, are cut through their openings into the pharyngeal pouches, 27 

 and 28, which are colored red. These two openings are the two gill-slits, 

 and in cuts P, Q and R their edges are indicated by dotted lines. Com- 

 parison of the figures will show that they soon undergo a very great 

 increase in size, until at the stage of cut S they include all the dorsal sur- 

 face of the aboral end of the pharynx except a strip, o, on the middle line. 



Each perithoracic vesicle, after it has established its communication 

 with the pharyngeal tube, gives rise to a diverticulum, cut P, which 

 grows inwards towards the middle line of the dorsal surface, as is shown 

 in Plate XXV, Fig. 7, H-H', g and h, and these diverticula ultimately 

 meet and unite on the middle line, as is shown in cut Q, to form the 

 cloaca, g'". Plate XXXIII, Fig. 3, K-K' shows, colored green, these two 

 diverticula meeting on the middle line of the dorsal surface, which is the 

 lower surface in the sections. 



Soon after the cloaca is thus formed its dorsal wall unites with the 

 ectoderm on the middle line, and the cloacal aperture, G* of cut S, is 

 formed at the point where this union has taken place. Plate XXXVI, 

 Fig. 5, g v shows the union between the dorsal wall of the cloaca and the 

 ectoderm, and it is shown again more advanced in Plate XXXVIII, 

 Figs. 88 and 97, g\ and also in Plate VIII, Figs. 1 and 2, g\ 



(4) The gill. The gill of salpa, cut S, o, and Plate XXXVI, Figs. 5 

 and 6, o, is simply that part of the body cavity which is bounded ventrally 

 by the dorsal wall of the pharynx, dorsally by the ventral wall of the 

 cloaca, and at the sides by the gill-slits. 



(5) The digestive organs. The oesophagus, stomach and intestine are 

 formed by gradual specialization in the course of the tubular diverti- 

 culum which is shown at q in cut P, growing out from the posterior end 

 of the right pharyngeal pouch, 27. In cut P, its blind end is dilated to 

 form the stomach, and in cut Q, the intestine, p, is shown growing out 

 from the ventral side of the stomach and bending upwards towards the 

 dorsal surface, where in cut S it opens into the cloaca. 



(6) The pericardium and the heart. The pericardium makes its 

 appearance very early, cut 0, as a closed vesicle, F, external to the aboral 

 end of the right pharyngeal pouch, 27. As this pouch lengthens it 

 carries the pericardium back with it, cuts P and Q, F, and Plate XXIV, 

 Fig. 2, e, and as the digestive tract is formed it pushes the heart towards 

 the ventral surface, cut S, F, and Plate XXV, Fig. 4, H, e, and Fig. 7, F, 



