W. K. BROOKS ON THE GENUS SALPA. 129 



must have been acquired at some period in its history, and I do not see 

 that Uljanin comes any nearer to an explanation of its peculiarities by 

 tracing it back to the Ascidians along an independent line. So far as I 

 know, nothing in the history of the Ascidians helps us to understand 

 these peculiarities of Salpa, and Uljanin's view accomplishes nothing 

 except to force him to seek for a secondary explanation of the con- 

 spicuous and undeniable resemblances between Doliolum and Salpa. 



As Grobben says, pa,ge 67, the shape of the body in Salpa and 

 Doliolum, the situation of the mouth and atrial aperture at opposite 

 ends, the arrangement of the muscle bands around the barrel-shaped 

 body, and the free-swimming habit, are in themselves very conclusive 

 evidence of their affinity, and while it is quite true that, as Uljanin 

 points out, these adaptations to similar conditions of life might have 

 been independently acquired, there is no good reason for thinking that 

 this has happened, for they exhibit both superficial and fundamental 

 similarity of structure. Uljanin indeed holds that the gills of Salpa are 

 not homologous with those of Doliolum. If by this statement he means 

 that the rod which usually, in Salpa, goes by the name of " gill " is not 

 the same thing as the branchial slits or stigmata of Doliolum, his objec- 

 tion is unworthy of consideration, for no one has ever seriously proposed 

 any such homology, although the " gill " of Salpa has its homologue in 

 Doliolum, as in Pyrosoma and the Ascidians, in the dorsal lamella. 

 If he means that the two apertures by which the pharynx of Salpa com- 

 municates with the atrium are not homologous with the branchial slits 

 of Doliolum, I can only quote my observations, already detailed, which 

 show that the gill-slits of Salpa are strictly homologous with those 

 of the Ascidians. I shall soon examine Uljanin's statement that the 

 atrium of Doliolum is not homologous with the atrium of Ascidians. 



SECTION 4. Salpa and Pyrosoma. 



I think that we may safely assume, as the first step in our com- 

 parison, that Doliolum and Salpa are closely related, and we come now 

 to the question whether Pyrosoma is closely related to Doliolum and 

 Salpa as Grobben believes, or is a swimming Tunicate of very different 

 origin, as Herdman and Uljanin believe. 



Since it was first pointed out by Huxley (Remarks upon Appendi- 

 cularia and Doliolum, p. 602), the affinity between Pyrosoma and 

 Doliolum has received general acceptance, and Grobben, p. 68, has 

 shown that there is a very close anatomical agreement between them. 



