REPORT ON THE KERATOSA. 9 



conclusion, as stated before, is founded on actual facts ; as to the first, I have no decisive 

 proofs for it, but I believe it will yet be adopted, at least provisionally, as a hypothesis 

 of comparatively great probability. That it is so, when compared with that of Dr. v. 

 Lendenfeld, is evident, since it does not, as does his theory, contradict the actual facts ; 

 that, again, the intussusception theory of Prof. Schulze ^ is not plausible has been indicated 

 by Dr. v. Lendenfeld ^ himself. 



The foregoing remarks have had two distinct aims : first, to give a plausible explana- 

 tion of the phenomenon of the formation of the horny fibres ; and second, to show that 

 this phenomenon is the same with regard both to the homogeneous and heterogeneous 

 fibres. Whether I have succeeded in my first task will be shown by later investigations ; 

 at any rate we must assume that the elements forming the homogeneous horny skeletal 

 fibres are just the same as those secreting the heterogeneous fibres ; and since in both 

 cases no special functional transformations of any of these elements take place, we must 

 come to the conclusion that, in thorough harmony with the fact that homogeneous and 

 heterogeneous horny fibres difier from one another only quantitatively, the development 

 of both kinds of fibres admits also of only relative distinction ; and that, accordingly, the 

 subdivision of the Keratosa into two groups, the one characterised by homogeneous, the 

 other by heterogeneous, skeletal fibres, would be thoroughly artificial. Whether such a 

 subdivision may be made according to the structure of the canal system will be discussed 

 later on. I proceed for the present to treat of the modifications of the skeleton, and now 

 pass on to those influenced by the tendency of most of the Keratosa to take up foreign 

 bodies into their skeletal fibres. 



As is well known, this tendency is characteristic only of Keratosa with homogeneous 

 skeletal fibres. Beginning with forms like most Coscinodermata and Hipjjospongioe, 

 whose fibres contain foreign bodies only exceptionally, here and there a sand-grain or frag- 

 ment of a spicule, going on to forms like many representatives of the genera Euspongia 

 and Ccicospongia, whose primary fibres are full of foreign enclosures, but the secondary 

 ones in most cases quite free from them, and, further, passing by forms like Psa'nwiocle7na 

 vosmaeri or Spongelia avara, both kinds of fibres of which are overcharged with foreign 

 enclosures, we come to the genus Psammopemma, characterised by an entire absence of any 

 fibres, the supporting skeleton consisting of sand-grains, portions of Foraminiferal shells, 

 fragments of spicules, &c., all lying separately, the secretion of the horny substance being 

 reduced to the formation of a thin horny envelope around each foreign body. 



To this tendency, again, a high systematic importance has been ascribed. Gray ^ and 

 Marshall * characterise their family of Dysideidse mainly by the richness of their fibres 

 in foreign enclosures. The systematic application of this character plays also a great 



1 Zeitschr. f. iviss. Zool., vol. xxx. p. 403. ^ Loc. cit, p. 291-292. 



2 Froc. Zool. Soc. Land., 1867, p. 503. ■• Zeitschr. f. wiss. Zool., vol. xxxv. p. 92. 

 (zooL. CHALL. EXP. — PART X.XXI. — 1884.) Hh 2 



