74 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



vertical. It is at first simply a furrow, and its upper and lower ends 

 remain permanently open, but in its middle region, Fig. 3, more magni- 

 fied in Fig. 9, its lips fold in and meet so as to convert it into a tube. Its 

 lower opening is shown at e in Fig. 5, and more enlarged in Fig. 10, and 

 its upper opening at e in Fig. 1, more enlarged in Fig. 6. The upper 

 end of the heart, Plate XXXV, is situated at the base of the gill, o, which 

 is a tubular rod bathed on all sides by the water which circulates through 

 the pharynx, c, and cloaca, g'". In the mature salpa the lower end of the 

 heart communicates with a large blood space or vessel which runs along 

 the middle line of the ventral surface under the endostyle. This vessel 

 is shown in section, not marked by a letter, in Plate XXI, Fig. 7, and 

 also in Plate XVI, Fig. 4. It is also shown in a young chain-salpa, in 

 Plate XXXVIII, Fig. 29, atj. In the young solitary embryo it is very 

 short, and it opens into that part of the body cavity which lies over and 

 around the placenta. 



Plate XXI, Fig. 1, cuts the heart, e, and the pericardium, /, of the 

 solitary embryo. Fig. 2 shows on the side next the stolon the epithe- 

 lium of the pericardium and that of the heart in contact with each 

 other. Figs. 3, 4, 5 and 6 pass through the inferior opening of the heart, 

 and Fig. 7 cuts the ventral blood-vessel. 



If the upper blood space of the stolon, j, of Fig. 7 be traced back 

 through sections 7, 6, 5, 4 and 3, it will be seen that it communicates with 

 the opening of the heart of the embryo, and through the ventral blood- 

 vessel, with the placental portion of its body cavity, and it is therefore 

 clear that it stands in very intimate relation to the two sources of nutri- 

 tion, the degenerating eleoblast and the placenta, as well as to the gill. 



When the heart of the chain-sa-lpa is beating in one direction, blood 

 flows from the eleoblast into the lower tube of the stolon out to its tip, 

 and back to the heart through the upper tube. In the reversed circula- 

 tion, blood from the gill of the embryo is driven by its heart into the 

 upper tube, and out to the tip of the stolon and back to the eleoblast. 



In the mature stolon, Plate XXXIV, Fig. 1, each blood tube has its 

 own endothelial lining, which lies in such close contact with the other 

 organs of the stolon that it easily escapes observation at all points 

 except the places where it bridges over the gap between the ectoderm 

 and the endodermal tube, Fig. 3, or in the angles at the sides of the nerve 

 tube. It is shown in Plate XXIII, Fig. 10, j, and in Plates XXIV and 

 XXX. 



