98 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



partially hidden in a proximal or dorsal view, like the one in the dia- 

 gram. 



These two structures, the rudimentary digestive tract, q, and the 

 rudimentary pericardium, e, are also shown in Plate V, Figs. 2 and 3. 

 As these figures are not diagrams but reconstructions from actual 

 sections, they are complicated by the secondary changes which have 

 been left out of the diagrams, and Fig. 3 represents a salpa which is 

 about to move to the right, and Fig. 4, one a little older, which is to 

 go to the left, and it will be seen that in both cases the digestive tract 

 and pericardium arise on the right, and in relation to the right pharyn- 

 geal tube, whether the salpa is to be a right-handed one or a left-handed 

 one. In Plate XXVII, Fig. 1, C, and Fig. 2, C, the pericardium, e, and the 

 digestive tract marked g, are shown, and it will be seen that the diges- 

 tive tract is proximal or dorsal and the heart distal or ventral. Through 

 an oversight, the rudiment of the digestive tract has been colored green 

 and marked g, like the right perithoracic tube in the other figures. 



The relations of the various structures in the diagram, cut O, will be 

 made clear by comparison with the following sections, which are, mor- 

 phologically, in the planes indicated by the horizontal lines. 



Plate XXXI, Fig. 3, B-B' passes through the ganglion and the oral 

 ends of the pharyngeal pouches. Fig. 2, B-B' cuts first, on the left, the 

 left pharyngeal tube in its fold of ectoderm, next the blood space, j, 

 through which the body cavity of the salpa communicates with those of 

 adjacent salpae. Plate XXX, Fig. 3, B-B shows the opening by which 

 the right pharyngeal tube, 27, communicates through the endodermal 

 tube, d', with the left pharyngeal tube, Fig. 2, B', 28, and also with the 

 pharyngeal tubes of the adjacent salpae. In Fig. 1 the body cavity com- 

 municates with the lower blood-tube, i, and the pharyngeal tubes are 

 cut below the level of their openings into the endodermal tube. In Plate 

 XXIX, Figs. 1 and 2, the right half of the body cavity is cut below the 

 lower end of the right pharyngeal pouch, while the left is shown at B'. 

 In Plate XXVIII, Figs. 1 and 2, B', the gill-slit through which the left 

 perithoracic vesicle, h, communicates with the left pharyngeal pouch, 28, 

 is shown ; while the right half of the body cavity, B, is empty in Plates 

 XXVIII and XXIX, and in Plate XXX, Fig. 2, B, the right perithoracic 

 vesicle is shown, at g, as yet without an opening into the right pharyn- 

 geal pouch. Salpa B-B' in these figures is a left-handed one, and the lack 

 of symmetry on its two sides is the result of secondary changes, although 

 the sections are still sufficiently like the diagram to be useful for com- 

 parison. 



