SYSTEMATIC POSITION OF FOEBESELLA TESSELLATA. 281 



four folds in their line of descent, whereas the Cynthias acquired a greater 

 number. In addition there is the further characteristic derived from the 

 condition of the tentacles, which remained simple in Styelids and became 

 compound in the Cynthiidse. In my opinion, then, we are justified in con- 

 sidering that a species which presents us with only four folds on each side 

 must be excluded from the genus Cynthia or Pyura ; but as it has compound 

 tentacles it must equally be excluded from the genus Styela. Therefore I 

 would place both Forbes's tessellata and Micbaelsen's stubenrauchi, if the latter 

 has undoubtedly not more than eight folds, together in the genus Forhesella, 

 occupying an intermediate position between Styela and Cynthia, and repre- 

 senting an early offshoot from the ancestral Cynthiids after they had attained 

 compound tentacles, but before the number of folds in the branchial sac had 

 increased beyond a total of eight. 



I have now before me a series of specimens of Forhesella tessellata, obtained 

 during the cruise of the ' Runa,-* from five localities on the west of Scotland^ 



VIZ. 



(1) Loch Sunart, 20 fathoms. 



(2) OfE Island of Canna, 60 fathoms. 



(3) South of Neist Point, Skye, 42 fathoms, 



(4) Off the Croulin Islands, 30 fathoms. 



(5) Inner Sound north of Croulins, 50 fathoms. 



This series demonstrates clearly the very definite nature of the flat plates 

 or scales into which the test is modified, and which are much more perfect 

 and regular than the rough tubercles or surface elevations found in some 

 species of Cynthia. I give (PL 19. fig. 4) a tracing with the "Edinger-"' 

 drawing-apparatus to show the very definite polygonal shapes and arrange- 

 ment of these plates from one of my Hebridean specimens. 



The internal structure of these northern individuals agrees,, so far as I have 

 examined it, very closely with that of specimens from the Isle of Man. The 

 branchial sac again, as in the former specimens, shows some variations ; but 

 in all cases the folds never exceed four on each side, which, taken alono- with 

 the compound tentacles, in my opinion places this curious little form in an 

 intermediate position between -Cynthia and Styela and entitles it to rank as a 

 distinct genus, Forhesella, in the subfamily Cynthiinse. 



V. Variation in Ascidia. 



The large specimens of Ascidia mentula, Miill., and Ascidia venosa, Miill., 

 which came up in the dredge on various occasions suggested to me that it 

 might be useful to examine the individual variation in some of the internal 

 organs which are usually described among the specific characters of these 

 two well-marked species. 



LINN. JOURN. ZOOLOGY, VOL. XXXU. 24 



