124 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



to the pharynx, and unite with each other to form the median atrium 

 or cloaca, which opens to the exterior on the dorsal middle line through 

 the atrial aperture. This aperture is, morphologically, the two original 

 spiracular openings of the perithoracic invaginations, and in most cases 

 these openings move towards each other until they meet and unite to 

 form the single median atrial aperture. In some cases, however, they 

 close up and disappear, while the atrial aperture breaks through as a 

 new opening on the middle line. In the latter case, however, it is plain 

 that we have to do with secondary changes, and the replacement of a 

 circuitous ancestral history by a more direct mode of development. 

 This secondary history is exhibited by Salpa, as I have shown already. 

 To recapitulate briefly, we have in the embryo of the solitary form, cut 

 A, p. 29, first, a pair of lateral perithoracic involutions from the surface 

 of the body. Then, as Plate XII shows, these involutions, Fig. 1, g", 

 extend towards the middle line and meet to form the median atrium, 

 Fig. 2, g"', and also extend downwards towards the rudiment of the 

 pharynx as a pair of perithoracic tubes, Fig. 4, g' and Fig. 5, g, which, 

 at the stage shown in Plate XII, end blindly. Cut B on p. 29 shows 

 these structures at this stage in a vertical transverse section. 



In the embryo which is shown in Plate XIII, the spiracular openings 

 of the perithoracic tubes have closed, and the external portions of 

 these tubes have moved towards the middle line, Fig. 6, g", where they 

 meet above the median atrium, Fig. 7, g'", from which the perithoracic 

 tubes, Fig. 8, gr iy , are continued down towards the pharynx, although 

 they still end blindly, as the vertical section of the same embryo, on 

 p. 30, shows. 



In the embryo shown in Plate XIV they open into the pharynx by 

 a single large aperture or gill-slit, g", on each side of the middle line. 

 In this plate, Figs. 3 and 4 cut the perithoracic tubes above the median 

 atrium ; Fig. 5 cuts the median atrium ; Figs. 6 and 7 cut the tubes at 

 lower levels, and Figs. 8 and 9 pass through the pharynx, c. Cut D on 

 p. 31 is a vertical section of the same embryo. Finally the median 

 atrial aperture, Plate XXXV, g", is formed as a new opening on the dorsal 

 surface in the way which is shown in Plate XVII, Figs. 6 and 7. 



In the aggregated salpa the atrial structures are formed in essen- 

 tially the same way. On each side of a cross-section of the stolon, Plate 

 XXXIV, Fig. 1, there is a perithoracic tube, g and h, which probably 

 arises at the base of the very young stolon, Plate XX, Fig. 3, as an 

 involution of the ectoderm, although as Plate XXI, Fig. 7 shows, the 



