REPORT ON THE TUNICATA OF PLYMOUTH. 79 



be by no means rarely incomplete in portions of the branchial sac ; 

 they are then represented by structures which could well be described 

 as "hooked fleshy tubercles." 



I will not maintain that this new view of Forbes and Goodsir's 

 very "remarkable invertebrate" is probable, but it is at least 

 possible. If it should prove eventually to be correct, a very in- 

 teresting connection between Diazona violacea and Tylobranchion 

 speciosum will have been established. 



By admitting the above-named differences between the branchial 

 sacs of Diazona violacea and Syntethys Hebridicus, it will be noticed 

 that I do not accept Alder's identification of his Guernsey specimens 

 of Diazona with Forbes and Goodsir's species. From Alder's account 

 I have been led to believe that he assumed this identity too hastily. 

 He states that his specimens were " at once recognised as the Syntethys 

 Hebridicus of Forbes and Goodsir," and upon this assumption he 

 endeavoured to find out what structural differences there might be 

 between this form and the Diazona violacea so admirably described by 

 the great French anatomist. His researches were not very fruitful of 

 result: "The only difference I can find is that the papillee of the 

 branchial sac in the latter [Syntethys Hebridicus) are stout and obtuse, 

 very different from the slender pointed form represented by Savigny ; 

 I have therefore determined to consider them distinct until further 

 observations decide the point." 



Now Alder's Guernsey specimens are certainly identical (specifically) 

 with the forms investigated by myself, and they are both from prac- 

 tically the same region of the English Channel ; there is further no 

 appreciable difference between the Plymouth forms and Savigny's 

 species. Therefore Alder's examples must also be referred to the 

 species Diazona violacea. 



The absence of any indication in Alder's paper that he re-examined 

 the " portion of a specimen (of Syntethys Hebridicus) from the 

 original habitat " which Professor Goodsir sent to him, renders in- 

 telligible what would otherwise have been a very strange omission on 

 his part. I refer to his failure to notice the remarkable discrepancy 

 between the structure of the branchial sac in the Channel specimens 

 and that described for Syntethys Hebridicus. 



Thus, although I quite agree with Alder that there is as yet no 



