W. K. BROOKS ON THE GENUS SALPA. 293 



with other tunicates, or even for any fundamental unity in his accounts 

 of the various species of Salpa, and his papers contain internal evidence 

 that he has misinterpreted his observations. 



Uljanin holds (7), p. 58 and p. 57, that the perithoracic structures of 

 doliolum are not homologous with those of the ascidians, but I have 

 already shown, page 130, that this view is not proved by his account 

 of their development, even if we admit that he has traced their whole 

 history in doliolum. 



Salensky believes that the cloaca and "gill" of salpa are not homo- 

 logous with the similar organs of the ascidians. 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 independent chamber, but a part 

 of the pharynx which is separated off by these folds. A careful study of 

 his description, especially pages 119, 200, 224, 225, and 229 of his first 

 paper (2), and pages 114, 139, 160, 163, 338, 339, and 354 of his second 

 paper (5), shows that his views not only involve this conclusion, but 

 that, if accepted, they would also force us to believe that the " gill " and 

 cloaca of one species of salpa are not homologous with the same struc- 

 tures in another species ; for his account of their origin in Salpa demo- 

 cratica and Salpa pinnata has almost nothing in common with his 

 account of them in Salpa af ricana, Salpa pectinata 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 which soon meet and unite above the gill to form the 

 atrium. 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 Salpa 

 democratica, the only point of agreement with my own observations on 

 Salpa pinnata is his statement that the gill is at. first solid, and that its 

 central cells are set free as development progresses. 



In his second paper (5), page 139, he retracts this statement, and 

 says that while his studies of the embryo of Salpa pinnata have in other 

 respects confirmed his account of the origin of the atrium and gill of 

 Salpa democratica, they show that the gill is in its origin a hollow 

 diverticulum from the body cavity. 



