REPORT ON THE TUNICA.TA OP PLYMOUTH. 77 



Colony massive, irregularly rounded, attached by a short, thick 

 pedicle or base ; total diameter about 7 inches, total height 5 or 6 

 inches ; of apple-green colour when alive, semi-transparent. 



Zooids often 2 inches long, with oral and cloacal orifices each six- 

 rayed. 



Branchial sac with sixty to eighty transverse rows of stigmata ; 

 meshes each containing three, rarely four stigmata ; internal longi- 

 tudinal bars for the most part completely formed ; but here and there 

 represented by T-shaped interserial papilla3, as in Tylobranchion ; 

 dorsal tubercle a large deep groove, elongate antero-posteriorly, with 

 thickened walls. 



Habits. — Attached to rocks and stones in deep water. 



Dredged at Plymouth on rough ground off Stoke Point, and off the 

 Eddystone in 20-40 fathoms of water. 



There are two remarkable statements in the original description 

 of the structure of Syntethys Hebridicus by Forbes and Goodsir which 

 have not, to my knowledge, received the attention which they deserve. 

 They are involved in the following account given by these naturalists 

 of the branchial sac in their specimens : 



"Branchial chamber with thirteen transverse rows of oblong 

 openings, fringed with ciliated epithelium ; hooked fleshy tubercles at 

 the intersections of the branchial meshes, each mesh presenting one of 

 the ciliated openings ; the tubercles give the internal surface of the 

 chamber a dotted appearance." (Trans. Roy. Soc. Edin , 1853, p. 

 307, cf. also for Forbes and Hanley, 1. c, p. 244.) 



Now, in the specimens of Diazona violacea dredged at Plymouth, 

 the number of transverse rows of stigmata greatly exceeds that given 

 by the eminent naturalists who described Syntethys Hebridicus; the 

 number is usually about sixty, seventy, or even more ! Further, the 

 stigma in each mesh are invariably three or four, the latter number 

 agreeing with the description and figure given by Savigny. 



Were Professors Forbes and Goodsir mistaken 1 Such a theory is 

 uulikely, for one of their figures (1. c. pi. ix, fig. 4 d) shows in outline 

 some of the appearances which they recorded in the words quoted 

 above. Indeed, this figure is too precise to admit of any doubt as 

 regards the approximate number of transverse bars (and, therefore, 

 rows of stigmata) in their specimens, and a difference in this respect 



