64 REPORT ON THE TUNIC ATA OF PLYMOUTH. 



whicli have uot^ 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 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 Hehridicvs ; 

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

 ther, the stigmata 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 ? Such a 

 theory is unlikely, 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 between Diazona violacea and Syntethys 

 Hehridicus must, I think, be admitted. 



But the more remarkable statement is that '' each mesh presents 

 one of the ciliated openings." That Forbes and Goodsir should 

 have made a mistake in the observations which gave rise to this 

 statement seems inconceivable, but it is surprising that they pass 

 no reflection upon so unusual a condition of the branchial sac. 

 There was plainly no error in the identification of the '' meshes," 

 for '' hooked fleshy tubercles " are stated to be present at the 

 " intersections of the branchial meshes " (a somewhat confused but 

 quite intelligible statement). Still, the fact of one stigma alone 

 being included in each mesh has either to be accepted or explained 

 away."^ 



It is conceivable that the appearance of one " ciliated opening " 

 corresponding to each mesh was due to a great transparency of the 

 "trame fondamentale" of the branchial sac, and that while the meshes 

 were observed, the true stigmata were not noticed ; but I cannot 

 reconcile this hypothesis with the assertion, so definitely made, that 

 the " oblong openings " were '' fringed with ciliated epithelium." 

 It is also impossible, and for the same reason, to imagine that the 



* This condition exists in Polyclinum sabulosum (Lahille, Comptes Rendus, cii, p. 1574), 

 and is approached in TylohrancMon speciosum. 



