166 Dr. W. A. Herdman on Diazona and Syntethys. 



Alder * in 1863 placed Forbes and Goodsir's species in the 

 genus Diozona under the name of D. hehridica^ and showed 

 that it did not differ from Savigny's form in colour, since its 

 living apple-green tint changed to violet on preservation in 

 alcohol. He also noted that the apertures of his specimen 

 from the Channel Islands were obscurely six-lobed, and thus 

 brought the descriptions of the two forms so closely into 

 accord that most subsequent writers have considered them to 

 be the same species of Diazona^ and the name hebridica has 

 dropped out of use. Diazona violacea has since been found 

 by Delia Valle f and others in the Mediterranean, by Giard \ 

 off the south-west coast of Brittany, by Lahille § off the north 

 coast of Britanny and the Mediterranean coast of France, and 

 by Garstang near Plymouth. 



Lahille has recently {loc. cU. 1890) given a detailed 

 description, and has shown that there may be as many as one 

 hundred rows of stigmata in the branchial sac, that there are 

 twenty-four tentacles, and that no true papillaj are placed at 

 the angles of the meslies. His figures 136 and 137 show 

 some meshes containing one, two, and three stigmata each. 

 Finally, Garstang (1891), although admitting the generic 

 identity of Diazona and Syntethys^ tries to show that Alder's 

 specimens from Guernsey were probably not identical with 

 Forbes and Goodsir's species, and that the latter may be 

 distinct from D. violacea. He points out the difference in 

 the branchial sac between his own specimens from Plymouth, 

 which he identifies as D. violacea^ and the figures and descrip- 

 tion given by Forbes and Goodsir — the latter showing only 

 thirteen rows of stigmata and only one of the stigmata in each 

 mesh. Upon these and the other characters given by Forbes 

 and Goodsir he redefines the species Diazona hebridica^ but 

 concludes by saying that " the whole matter is so beset with 

 doubts that it is greatly to be desired that specimens should 

 be obtained again from the Hebrides, and their anatomy 

 redescribed " (foe. cit. p. 66). On reading this last sentence 

 I at once remembered that I had in my collection a Hebridean 

 specimen of Diazona dredged off the north coast of Mull in 

 1885 by the Duke of Argyll, and sent to me for identification 

 through Dr. John jNLurray. I had examined the specimen in 

 1885, identified it as D. violacea^ made some microscopic 



* Ann. & Mag. Nat. Hist. (3) vol. xi. p. 169._ 



t ' Coutiib. alia Storia natuvale delle Ascidie composte del Golfo di 

 Napoli,' 1877, p. 10 ; 'Nuove Contribuzioni,' 1881; and ' Sul Kiiigio- 

 vaoimento &c.,' 1884. 



\ ' Comptea Rendus,' ciii. p. 755 (1886). 



§ ' Recherches sur les Tuniciers/ &c., 1890, p. 257. 



