130 Mr. W. K. Brooks on the 



tAvo tubes soon lose tiieir communication with the exterior, 

 and the median cloacal aperture is an independent opening 

 which is formed later. After the pharynx is formed each 

 perithoracic tube unites with it to form a gill-slit. Finally, 

 after the perithoracic system is completely outlined its follicu- 

 lar cells degenerate and are gradually replaced by blastomeres. 



Our knowledge of the perithoracic system of Salpa in both 

 the solitary and the aggregated form is in great confusion. 



Salensky has described the origin of the " gill " and of the 

 median atrium or cloaca of the embryo in a number of species ; 

 but the reader of his papers will search in vain for any basis 

 of comparison with other Tunicata, or even for any funda- 

 mental unity in his account of the various species of Salpa, 

 and his papers contain internal evidence that he has misin- 

 terpreted his observations. 



IJljanin holds that the perithoracic structures of Doliolum 

 are not homologous with those of the ascidian, and Salensky 

 holds the same view regarding Salpa. He says that the 

 " gill " is part of the body-cavity which is shut in by folds in 

 the walls of the pharynx, and that the cloaca is not an inde- 

 perdent chamber, but a part of the pharynx which is shut off 

 by these folds. A careful study of his description, especially 

 pages 119, 200, 224, 225, and 229 of his first paper, and 

 pages 114, IS 9, 160, 163, 338, 339, and 354 of his second 

 paper, will show that his views not only involve this conclu- 

 sion, but that they would also force us to believe that the 

 " gill " and cloaca of one species of Salpa are not homologous 

 ■v\ ith the same structures in another species ; for his account 

 of their origin in Salpa democratica and Salpa pinnata has 

 almost nothing in common with his account of them in Salpa 

 ofricanOj Salpa pecttnafa, and Salpa fusiformis. 



In his first paper on Salpa democratica he says that, like 

 Leuckart, he regards the gill as part of the inner mantle or 

 branchial sac, that in origin it is nothing more than a strongly 

 developed ridge or thickening on the middle line of the dorsal 

 surface of the pharynx, and that on each side of it the cavity 

 of the pharynx is pushed upwards to form a pair of pouches, 

 Avhich soon meet and unite above the cloaca. In this way 

 the gill-ridge is transformed into a rod, and the rod, which is 

 at first solid, becomes tubular by the conversion of its axial 

 cells into blood-corpuscles. 



In this account of the origin of the perithoracic structures of 

 Sulpia democratica the only point of agreement with my own 

 observations on Salj)a pnnnata \^ his statement that the gill 

 is at first solid, and that its central cells are set free as deve- 

 lopment progresses. In his second paper he retracts this 



