34 THE VOYAGE OF H.M.S. CHALLENGER. 



tive thickness of the horny walls of its fibres — is greater than that between Verongia 

 and A'plysina. 



Verongia. 



On page 7 I have stated the grounds which make it, though not very probable, 

 yet not impossible, that the Lvffari(S have perhaps nothing to do with AplysinidfB, 

 being only specifically modified Cacospongm. At any rate — though bearing in mind 

 the necessity for certain concessions to the present state of our knowledge, we must 

 necessarily place the genus in the family Aplysinid^ — the genus, owing to the fact 

 that many of the smaller fibres of the skeleton of some of its representatives must 

 be called homogeneous, is so very closely connected with true Cacospo7igi(B that its 

 diagnosis can only be of a very conditional character. The same must be said with 

 respect to the mutual relations of the genera Verongia, Bowerbank,^ and Aijlysina. 

 Hyatt characterises his Aplysinidse " by the regular net-like anastomosis of the fibres, 

 the tendency of this to occur in the same plane, the flatness of the fibres, and the thinness 

 of their walls." ^ He characterises his Dendrospongiadse, of which the genus Verongia 

 is a representative, " by the irregular anastomosis of the fibres of the skeleton, by their 

 rotund form, and by the thickness of the horny walls." ^ As to the regularity or irregu- 

 larity of the network of the skeletal fibres, the conditional nature of this character is but 

 too evident ; — it is well known what approximate expression geometrical outlines find 

 in organised beings ; again, as to the flatness of the fibres of Hyatt's Aplysinidre, this 

 character seems even more doubtful. F. E. Schulze has not only ascertained that the 

 fibres of Aplysina acroplioha are all more or less mathematically cylindrical and their 

 transverse section circular, but has also made it probable that the above-mentioned state- 

 ment of Hyatt is due to the circumstance that the specimens he had for examination were 

 dried, in which state all thin-walled round tubes filled with a fluid mass shrink, and form 

 compressed tubes with transverse sections of elliptical outline.* Finally, so far as the 

 comparative thickness of the fibre-walls is concerned, many of the foregoing lines have 

 been written precisely to show that there are thick-walled fibres which show a tendency 

 to become thin-walled, and again, in the Aplysince, fil:)res with the contrary tendency ; 

 we see therefore that the three characters in cpiestion — provided that Hyatt is correct as 

 to the conjectural flatness of the skeletal fibres of the Aplysinidce — are of a thoroughly 

 conditional nature. 



Ap)hjsina. 



The diagnosis of this genus having been already given when speaking of the j^receding 

 genus, all existing genera of the Keratosa have been presented to the reader, and I 



' Monogr. ot Brit. Spong., vol. i. p. 209 ^ Revision, &c., vol. i. p. 404. 



2 Revision, &c., vol. i. p. 400, * Zeitschr. f. vdss. Zool., Bd. xxx. p. 39!). 



