34 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



In Plate XLV, Fig. 8, v is the cellulose mantle which is not yet per- 

 forated, and c is the cavity of the pharynx. The ectoderm and endoderm 

 are united at the edge of the lower lip, but they are as yet separate in the 

 upper lip, which is rounded and thick and protuberant, with three trans- 

 verse muscles, w, and at a later stage the thin lower lip is tucked inwards 

 under the rounded upper lip, to form the oral valve of the branchial sac. 



The gill of salpa, Plate XLII, Fig. 8, o; Plate XVIII, Fig. 4 and Fig. 

 8, o, Plate XXXV, o, and Plate XLV, Fig. 4, o, is simply the space 

 bounded by the pharynx below, the gill-slits at the sides and the cloaca 

 above. It is at first a solid mass of follicle cells with blastomeres which 

 give rise to its lower or endodermal epithelium, but its sides are only 

 slowly covered with blastodermic epithelium, and its upper surface 

 consists of follicle cells in embryos which have acquired most of their 

 organs. The follicle cells in its cavity degenerate as the intestine extends 

 into the gill. 



The cloaca is at first lined throughout by somatic follicle cells. Some 

 of these begin to migrate into its cavity very early, and in some speci- 

 mens the whole chamber is so choked up with them that it is difficult to 

 trace it. The blastodermic epithelium gradually extends over its whole 

 surface, and the follicle cells degenerate and disappear. Its cavity then 

 enlarges very rapidly, as will be seen by comparing Fig. 8 of Plate XVIII 

 with Plate XXXV, or by comparing the embryos on Plate XLI. After 

 its epithelial lining is complete, the ectoderm, Plate XVII, Fig. 6, a, bends 

 downwards around a circular area to meet it, in such a way that a lump of 

 visceral follicle cells is shut in under an arched cover of ectoderm. These 

 cells then become vacuolated and finally disappear, as does also the cap 

 of ectoderm. The ectoderm around the edge of the circle now bends 

 inwards upon itself as shown in Fig. 7, and becomes continuous with the 

 cloacal epithelium, and all of this inside the circular line of adhesion 

 degenerates as shown in the figure, and finally disappears, to form the 

 cloacal aperture. Figure 6 of Plate XVII is from the embryo shown in 

 Plate XXXV, and Fig. 7 from one like Plate XLI, Fig. 3. 



It will be seen from this account that the cloacal aperture is a new 

 formation, and that it is not the two spiracles united into a single 

 aperture, although I believe that the history of salpa is quite reconcilable 

 with the view that it is phylogenetically a pair of spiracles. The spiracles 

 which are formed in the somatic layer of the follicle lose their external 

 openings, as we have seen, and the spiracular tubes move towards the 

 middle line and unite at the spot, above the cloaca, where the aperture is 



