NOTES ON VARIATION IN THE TUNICATA. 359 



one of the new Compound Ascidians, obtained during the 

 "Challenger" expedition, Tylohranchion speciosurn, from 

 Kerguelen Island, has a number of branched papillae on 

 the transverse vessels of the branchial sac, which are, I 

 believe, simply connecting ducts with rudimentary internal 

 longitudinal bars attached to them. Similarly, I am in- 

 clined to regard the small papillae which project from the 

 transverse vessels in Perophora listeri as being really 

 connecting ducts upon the ends of which internal longi- 

 tudinal bars might possibly have been developed. Figures 1 

 to 4 on Plate IX. shew a series of stages by which a complete 

 internal longitudinal bar (fig. 1, L I) might be reduced to 

 simple papillae, projecting from the transverse vessels at the 

 angles of a mesh (fig. 4, c. d). All these stages may be seen 

 as irregularities or variations in the branchial sacs of some 

 British Ascidians. 



In describing the shape of the meshes and the number 

 of stigmata they contain, it is necessary to avoid the edges 

 of the sac, since the dorsal and ventral rows of meshes 

 are usually very much larger than the rest, and sometimes 

 contain twice as many stigmata. In some branchial sacs the 

 stigmata are, as individual varieties, exceedingly irregular in 

 their arrangement, and this appears to be especially the 

 case where there are several orders of transverse vessels 

 present {e.g., large, small, and medium-sized vessels, 

 arranged alternately), some stigmata being twice or even 

 thrice as long as their neighbours (see PI. IX, fig. 5). 



The small transverse vessels (or horizontal membranes) 

 are very inconstant, and cannot be depended upon. In those 

 species where they occur, they may be present in one mesh, 

 dividing it horizontally into two parts, and absent in all the 

 neighbouring meshes, or they may be present in nearly every 

 mesh of the sac (PI. IX. figs. 5 and 6, tr"). In some cases, 

 they interrupt the stigmata, while in others the stigmata pass 



