SOME ASCIDIANS FROM THE ISLE OF WIGHT. 139 



After much consideration I have arrived at the conclusion that the 

 specimens whose structure I have just described represent the species 

 Ascidia depressa of Alder, and that the Ascidians described under this 

 name by Heller, Herdman, and Roule are distinct from it. 



A reference to Alder's original account will show how perfectly in 

 every point my specimens agree with his description, with the excep- 

 tion that I can make no statement as to the presence of red ocelli 

 around the apertures. I did not observe these spots in the living 

 animals ; but on the other hand I paid no attention to the point, and 

 probably overlooked their existence. In every other respect the corres- 

 pondence is complete, and I may draw especial attention to the following 

 details — the shape and colour ; the expanded edge ; the position and 

 form of the apertures ; the granulations (minute tubercles) on the upper 

 surface ; the absence of intermediate papillae in the branchial sac (for it 

 was Alder's habit to imply the absence of these structures when he 

 made no direct reference to them) ; and the size. 



If this be really so, it necessarily follows that Heller's specimen 

 described under the same name is distinct. The structure of the test 

 is alone sufficient to distinguish my specimens from his. The bladder- 

 cells in the former are huge, of spherical or frequently polyhedral form, 

 exactly as Heller has himself described and figured for his Ascidia 

 rudis (1. c.j p. 14, Taf. v, fig. 6) ; but for his A. depressa a very 

 different condition was described by him (1. c, p. 15). Further, 

 Heller's A. depressa was destitute of the superficial microscopic 

 tubercles which are present in my specimens (and in Alder's) and 

 which Heller himself also figured for another species (A rudis, 1. c). 



Secondly, the specimens which Prof. Herdman has referred to this 

 species differ from Alder's in possessing intermediate papillae on the 

 internal longitudinal bars ; Alder would certainly have noticed the 

 existence of such papillae as Herdman has figured (1. c, supra, pi. xviii, 

 fig. 4), if they had existed in his specimens. Prof. Herdman's speci- 

 mens cannot belong to the same species as these from the Isle of Wight, 

 because in the latter the internal longitudinal bars rarely show a 

 trace of intermediate papillae, except when the meshes have grown to 

 a size when they are almost twice as long as broad ; in Prof. Herd- 

 man's species these papillae are normally present, and the meshes are 

 elongated transversely. Further, the structure of the dorsal lamina 



