320 BULLETIN OF THE 



also been described as the body cavity or sinus system. The inner 

 tunic, or branchial sac, has been shown to he part of the inner or 

 digestive layer of the gastrula. The third tunic, now to be described, 

 is the atrial tunic, and its cavity the atrial chamber or cloaca. 

 The earliest stages in the formation of this tunic were not traced 

 in either form of Salpa, but as it presented, when first observed, 

 an appearance very similar to that described by Kowalevskv,* as 

 presented during the early stages in the development of Pyro- 

 soma, it is possible that it originates in the same way ; although 

 in the absence of any observations upon its origin, the fact that 

 Kowalevsky's observations upon" Pyrosoma are here referred to 

 must not be regarded as implying anything more than a belief that 

 the similarity which is known to subsist between the two genera, in 

 many other respects, may extend to the way in which the atrium Is 

 formed. In the egg-embryo of Pyrosoma two circular depressions 

 appear upon the neural side of the outer tunic, near the anterior end, 

 and these deepen so as to form tubes derived from the outer layer, 

 witli cavities which open externally ; these tubes lengthen and pene- 

 trate the body cavity upon each side of the branchial sac ; their exter- 

 nal openings close, and they become separated from the outer tunic, 

 and form the " lateral atria " of Huxley. f According to Huxley 

 (pp. 215, 21G), ■ — and Kowalevsky's account corroborates that of 

 Huxley in every particular, — these at first " arc very small and thick- 

 walled ; they soon become larger and the walls proportionally thinner, 

 and the sacs themselves are both absolutely and relatively larger. 



" In Fig. 29 they are very much larger and thinner, and their rela- 

 tions to other organs are especially worthy of attention. The outer 

 layer of each is applied to the outer tunic of its side, leaving a small 

 interspace, which communicates freely with the great posterior sinus, 

 in which the intestine and genatilia are disposed, and with the ante- 

 rior sinus which lies between the pharyngeal wall [branchial sac], 

 and the external tunic. This interspace is, in fact, the parietal sinus. 

 The internal layer, continuous with the outer anteriorly and poste- 

 riorly, but separated from it by a wide chamber fur the rest of its 

 length, is applied against the wall of the pharynx [branchial sac], for 



* Ueber die Entwickelungsgescliiclite cler Pyrosoma. Von A. Kowalevsky. Arcliiv. 

 fur Mikr. Anat., XII., 1S75, p. 597. 



t Huxley, Anatomy and Development of Tyrosoma, p. 205. Trans. Linn. Soc, 

 XX 1 1 1., 1800, p. 193. 



