128 SOME ASCIDIANS FROM THE ISLE OF WIGHT. 



mediate papillae in the brancliial 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 ruclis (1. c, 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 micro- 

 scopic 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 specimens cannot belong to the same species as these 

 from the Isle of Wight, because in the latter the internal longi- 

 tudinal 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. Herdman's species these papillas are normally 

 present, and the meshes are elongated transversely. Further, the 

 structure of the dorsal lamina is very different in the two cases. 

 Prof. Herdman in the same paper noticed the existence of tubercles 

 on the dorsal lamina of Ascidia pleheia, so that there is no reason 

 to suppose that he overlooked them in his A. depressa. 



Lastly, M. Eoule has described under the name Ascidia depressa, a 

 species which, while probably identical with Heller's, is undoubtedly 

 distinct from Alder's species. The mode of attachment, the shape 

 of the body, and the structure of the branchial sac are very different 

 in the two cases. The species described by Heller and by Roule 

 presents a close affinity with Ascidia mentula, and still more, perhaps, 

 with Alder's (not Heller's) Ascidia rudis ; but there is nothing in 

 Alder's description of A. depressa to indicate a similar relationship 

 for that species, and my specimens are distinctly against it. 



Ascidia depressa, as now re-described, is very closely related to 

 Traustedt's Ascidia (Phallusia) pusilla (Mitt. Zool. Stat. Neap., iv, 

 1883, p. 465, Taf. xxxiv, figs. 16, 17 ; Taf. xxxv, fig. 26). The chief 

 points of difference are found in the different proportions of the 

 length to the breadth of the body, the length of the siphons, the 

 breadth of the largest horizontal vessels of the pharynx, the 



