78 WALTER GARS TANG, M.A. 



between Diazona violacea and Syntethys Hebridicus 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 brancial 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 state- 

 ment). 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 almost impossible, and for the same reason, to imagine that the 

 meshes were totally devoid of stigmata, as in Pharyngodictyon 

 mirabile of the " Challenger" collection, described by Herdman.f 



I am obliged, therefore, to conclude that Syntethys Hebridicus 

 actually possessed, as Forbes and Goodsir stated it to possess, a 

 branchial sac containing about thirteen transverse rows of oblong 

 stigmata, and presenting a " hooked fleshy tubercle " at the junction 

 of every longitudinal and horizontal bar. 



It should be noticed that in the original description there is 

 nothing irreconcilable with the view that the branchial sac of 

 Syntethys Hebridicus may in reality have been quite destitute of true 

 internal longitudinal bars, and possibly of horizontal membranes; 

 the " hooked fleshy tubercles " may have been such rudimentary con- 

 necting ducts and bars as Herdman has described and figured for 

 Tylobranchion speciosum (1. c, p. 161). In this connection I may 

 state that I find the internal longitudinal bars of Diazona violacea to 



* This condition exists in Polyclinum sabulosum (Lahille, Comptes Rendus, 

 cii, p. 1574), and is approached in Tylobranchion speciosum. 



t Herdman, " Challenger " Keport, vol. xiv, pt. xxxviii, p. 155, 



