1G8 Dr. W. A. Herdman on DIazona and Sjntethys. 



twice as many stigmata present as are visible on the inner 

 surface. The stigmata behind the bars seem to me smaller 

 in size ; but this I am not certain about. In other parts of 

 this same sac I find meshes with two, three, or four stigmata. 

 In the Naples specimen close to the dorsal edge, where the 

 internal longitudinal bars are usually imperfect for as much 

 as eight or nine series of meshes, I find the papilliform con- 

 necting-ducts, which indicate the position of undeveloped bars, 

 placed one stigma apart, so that if meshes were formed there 

 they would contain each one stigma only. In the Hebridean 

 specimen there seem generally two or three stigmata in a 

 mesh, sometimes four or five, some parts of tlie sac being in 

 this respect exactly like Garstang's figure {loc, cit. pi. ii. 

 fig. 7). I have not noticed meshes containing one stigma 

 each so distinctly as in the Plymouth specimen, but I have 

 no doubt such might be found by examining a few other 

 ascidiozooids. 



Finally, the " hooked fleshy tubercles " of Forbes and 

 Goodsir's description can, as has been suggested before, be 

 quite satisfactorily accounted for by the corrugation of the 

 internal longitudinal bars, the thick prominent connecting- 

 ducts which seem to project on each side where they join the 

 bars, and the imperfect condition of the bars in some parts of 

 the sac. 



When a branchial sac is first opened and is examined in 

 water under the microscope the appearance of large papillae 

 at the angles of the meshes is so distinct that it is difficult to 

 realize, until the specimen has 'been stained, mounted, and 

 examined with a high powei', that only connecting-ducts and 

 more or less irregular bars are present. There is no difficulty 

 in understanding how some of the earlier investigators fell 

 into the error of supposing that they saw large papillai. 



I think, then, that all the supposed peculiarities of Syntethys 

 hebridica can be satisfactorily disposed of. Perhaps the only 

 point in Forbes and Goodsir's description which still requires 

 explanation is the thirteen rows of stigmata, and I can only 

 suggest that, if there was no mistake about the observation, 

 they may possibly have examined a young ascidiozooid with 

 rather a small branchial sac. Unless the branchial sac is a 

 fairly large one and is well spread out, it is only too easy to 

 miss a great many of the rows of stigmata. 



It is still, of course, open to any one to say that the 

 Hebridean specimen dredged by the Duke of Argyll is, as I 

 have shown above, Diazona violacea, but is not necessarily 

 Forbes and Goodsir's Syntethys hebridicus. This is con- 

 ceivable, but is not at all likely, since the specimens are prac- 



