REPORT ON THE KERATOSA. 79 



qviantity of tliem is, however, still to be found. Are these spicules to be regarded as 

 physiologically functional, however it may be ? Can we assume that a single row 

 of siliceous spicules render the horny fibres of Chalina limbata more hard ? Can we 

 ascribe to them any other than phylogenetic significance ? Of course not ; and, on the other 

 hand, it is also plain that a sudden apparition of spicules in the horny fibres previously devoid 

 of them is thoroughly inexplicable. On the contrary, the substitution of a horny skeleton 

 for a skeleton represented by siliceous spicules is very simply imaginable. In numerous 

 Silicea we have to deal with forms whose siliceous supporting apparatus forms throughout 

 a network in the proper sense of the word. In many other Silicea we meet with forms 

 whose skeleton is represented by spicules lying exclusively isolated ; there are again 

 amongst them forms whose skeleton is constituted by spicules aggregated in rows. That 

 a skeleton represented by a compact network is of a firmer build than a skeleton repre- 

 sented by spicules lying isolated is evident. From this point of view it becomes clear 

 that had a sponge' once received the faculty of cementing the rows of its spicules with 

 any connecting substance, this acquirement would have proved to be of a great profit to 

 it. It is also equally plain that if the cementing substance proved to be equally hard 

 and steadfast, and at the same time more elastic than the siliceous spicules, it might, in 

 the course of time, have forced away these latter entirely. I hope this theory gives a 

 very simple explanation of the phenomenon. It is also in harmony with the facts 

 concerning the structure of the soft parts of Keratose sponges, and, on the other hand, 

 of Chalinidae, Renieridse, &c. Of course the literature of the question is very poor ; 

 some remarks of Dr. Vosmaer^ on the structure of the Renieridse, as well as a couple of 

 observations on the anatomy of the Chalinidae by Dr. Keller f the observation of this 

 naturalist^ on the structure of Reniera semitubulosa executed under the influence of 

 Prof. Haeckel's statements on the non-existing racemose type of the canal-system being- 

 unreliable ; they are, however, quite sufiicient for our purposes, and with regard to the 

 Renieridse I can also make use of my own investigations. As to the Chahnidfe, the 

 drawing by which Dr. Keller illustrates the internal organisation of his Chalinula fertilis 

 {Joe. cit., pi. xviii. fig. 1) cannot be misinterpreted; the ground-mass being devoid of 

 any granules, and the flagellated chambers of special cameral canaliculi, it is clear that we 

 have here to deal with the type of the canal-system characteristic of Spongelidte, and the 

 size of the flagellated chambers being, according to Keller (loc. cit., p. 327), 0"02 mm. on 

 an average, and their shape round, with that special modification which distinguishes 

 my genus Psammoclema. As to the internal structure of Renieridse, I difi"er somewhat 

 from Dr. Vosmaer. Indeed, his remarks are extremely short. He states, however, 

 that their anatomical organisation recalls \'ividly that of Eiiplectella aspergillum, as 



' Voorloopig berigt omtrent h. onderzoek aan de Nederl. werktafel in h. Zool. Stat, te Napels, Ilaag (?), 1881. 

 2 Zeitschr. f. wiss. Zool, Bd. xxxiii. p. 326. ^ Ibid., Bd. xxx. p. 579. 





