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



the actual data, viz., to the differences in structure distinguishing the horny sponges from 

 other Silicea. The usual and natural characteristic of the Keratosa is the following: — 

 Porifera with horny skeleton devoid of proper (siliceous) spicules. This diagnosis alludes 

 to this, that there must exist amongst Silicea, sponges although provided with a true 

 horny skeleton, yet characterised by the possession of spicules produced by the sponge 

 itself This allusion concerns the groups of Silicea known under the name of Chalinidse, 

 whose main systematic character consists in the possession of a horny skeleton recalling 

 as to its external stracture that of true Keratosa, but rich in horny substance as it is, yet 

 containing within its fibres proper spicules enclosed. Now through the genus Chalinula, 

 0. Schmidt, the Chalinidse are most closely allied to ty^jical Monactinellida. There are 

 accordingly between a typical horny sponge and a typical Monactinellid long series of inter- 

 mediate connecting stages, and their existence proves that the Keratosa and Monactinellida 

 must have had the same phylogenetic origin. This has never been disputed ; and, on the 

 whole, it is in thorough harmony ^dth embryological data also. The larvae of Keratosa as 

 described by Barrois^ [Verongia [AplysiUaf] rosea) and F. E. Schulze [Euspongia 

 officinalis,^ Spongelia pallescens,^ A'phjsiUa sidphurea*) and those of Chalintda fertilis 

 and Reniera fiUgrana as described by Keller' and Marshall," as well as their pre^dous 

 and probably further development, admit of no absolute distinctions. Indeed, while the 

 usual mode of division of the ovum is equal, that of the ova of Clialinula is, according 

 to Keller, unequal. But, firstly, this diS'erence is of a very subordinate nature, and, 

 secondly, it is still questionable whether this statement of Keller is more rehable than 

 his suggestion as to the sexual dimorphism of the species in question. There can be, I 

 repeat, no doubt as to the Keratosa and Monactinellida having had the same origin. 

 But the matter, indisputable as it is, can be interpreted diS"erently. The genealogical tree 

 accompanying the paper of Prof Schmidt on the sponges of Algeria (Joe. cit., p. 35) shows 

 that this naturalist considers the Keratosa to be an older group than the Monactinellida, 

 to represent, namely, a group from which the true Silicea have originated. If this be true, 

 the systematic proceeding of Hyatt I have spoken of a couple of pages before would 

 receive a thorough sanction, and the class of Non-calcarea, Vosmaer (for in such a case the 

 designation of Silicea applied to the group by Gray would be no longer admissible), would 

 require to be subdivided into two orders, Keratosa and Silicea. This suggestion is, 

 however, far from being reliable, and a short deliberation renders it obvious. I ask 

 what appears more easily and naturally realisable, the transformation of a SiHceous into a 

 Keratose sponge, or vice versa of a Keratose sponge into a Monactinellid. I think there 

 can be no doubt as to the answer. In the species Chalina limbata, Bk., we have to do 

 with a sponge whose skeletal fibres are extremely poor in proper spicules ; an insignificant 



' Ann. d. Sci. Nat. (Zool), scr. 6, t. iii., 1876, p. 56. ^ Zeitschr. f. wiss. Zool., Bd. xx.\ii. p. 642. 



3 Ibid., Bd. xxxii. p. 144. * Ibid., Bd. .\sx. p. 414. 



5 Ibid., Bd. xxxiii. p. 317. « Ihid., Bd. xxxvii. p. 221. 



