W. K. BEOOKS ON THE GENUS SALPA. 115 



than undulations in the outline of the ectoderm. In the region of the 

 endodermal pouches, 27 and 28, the body cavity is very much more shut 

 in, and section 4 of Plate XV cuts the bottom of the left body cavity just 

 above the point where it begins to deepen, while the right half of the 

 section is at a lower level and shows the ectoderm as an undulating line. 

 Section 6 of Plate XV shows the left pharyngeal pouch, 28, and the left 

 perithoracic tube, h, shut in to a deep ectodermal fold, while on the right 

 side of the section the ectodermal fold is cut below the level of the 

 pharyngeal pouch. This section also shows the opening by which the 

 left pharyngeal pouch, 28, opens into the endodermal tube, d', and in 

 section 7 the opening of the right pharyngeal pouch, 27, as well as that 

 of the left, 28, is shown. Fig. 7 also cuts the pericardium, e, and the 

 right perithoracic tube, g, above its lumen. Fig. 8 is like Fig. 7 on the 

 right side, except that it passes above the pericardium, but on the left it 

 passes through part of the upper blood space, .;', and cuts the oral pro- 

 longation of the left pharyngeal tube above its connection with the endo- 

 dermal tube, d'. Fig. 9 passes above the left pharyngeal pouch, but it 

 cuts the right one close to its blind end. Finally, section 10 cuts the 

 ganglia, s, which are entirely shut in by the folds of ectoderm. 



The cuts N and 0, on page 80, and the series of sections in Plates 

 XXIII and XXIV, show the gradual extension of the pharyngeal pouches 

 towards the oral and aboral ends of the body. The oral ends, Plate 

 XXIV, Fig. 5, 27 and 28, have reached the level of the ganglion, and are 

 dilated and much larger than the tubes, Figs. 4 and 3, which connect 

 them with the region of each pouch which opens into the endodermal 

 tube, d', of Fig. 2. At this stage, as in the one before, all the salpae are 

 exactly alike and symmetrically placed with reference to the bilateral 

 plane of the stolon, but in the next stage, Plate V, Fig. 2, and in all the 

 following stages, the salpa3 which move to the right are different from 

 those which move to the left. 



Plate V, Fig. 2, shows a salpa which is to pass to the right, at the 

 stage in which the first traces of the secondary changes make their 

 appearance. Like all the other figures in this plate, it is a proximal or 

 dorsal view of a single salpa. 



The lower part of the figure is like Fig. 1 in all essential particulars, 

 but great changes have taken place in its upper part. As shown in 

 Plate XXXI, Fig. 3, B-B', the ganglion, s, has moved outwards and 

 downwards, and the oral ends of the two pharyngeal pouches have met 

 and united behind or distal to the ganglion, or on what is to become the 



