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



Considering as the skeleton — in the usual physiological sense — all those solid parts 

 of the animal body which serve as a supporting frame and as a protecting carapace, we 

 may point out, first of all, that the skeleton of the Deep-sea Keratosa in general is composed 

 of three very different portions, viz., (1) spongin-fibres, produced in the mesoderm of the 

 sponge, and characteristic of all true Keratosa ; (2) xenophya, or solid foreign bodies, 

 taken up from the bottom of the deep sea and disposed in the mesoderm ; (3) chitinous 

 tubes of Hydroids which live in symbiosis with the majority of our Keratosa. The two 

 latter elements of the skeleton may be better called pseudo-skeleton, since they are 

 foreign bodies not produced by the sponge itself; but they generally possess in our 

 Deep-sea Keratosa a far greater importance than the true skeleton of the sponge itself, 

 composed of its proper spongin-fibres. 



The first fact that strikes one in the examination of the Deep-sea Keratosa is the 

 circumstance, that in all cases by far the greatest part of the body is composed of various 

 xenophya, and not of the tissues and organs of the sponge itself. The foreign enclosures 

 are everywhere found in such large masses that their total volume is always far greater 

 than that of all the parts of the sponge proper together. The latter form often scarcely 

 one-third or one-fourth of the whole volume, or less ; whilst the xenophya occupy two- 

 thirds or three-fourths, or more. Comparing the weight of the two different body- 

 components, their disproportion, of course, appears far greater. The xenophya being- 

 much heavier than the delicate soft tissues of the sponge itself, the weight of the former 

 is probably usually more than 90 per cent., the weight of the latter less than 10 per 

 cent. 



The xenophyal skeleton is the only essential part of the skeleton in the two first 

 families, Ammoconidae (PI. VIII.) and Psamminidag (PI. VII.); whilst it is combined with 

 spongin-fibres, and with symbiotic Hydroid tubes in the two other families, Spongelidaa 

 (Pis. IV.-VI.) and Stannomidae (Pis. L— III.). But even in these latter the foreign 

 pseudo-skeleton, composed of the chitinous tubes of the symbiotic hydrorhiza, plays a 

 more important part than the true spou gin-skeleton of the sponge itself. 



The spongin-fibres in our Deep-sea Keratosa are constantly very thin and small, and 

 scantily developed, far less than in the well-known Keratosa of shallow water. In the 

 former are never found the stout and strong horny main fibres, which erect the firm 

 scaffold of the body in the latter. The place of these main fibres is taken by the 

 chitinous tubes of the symbiotic Hydroids, and this remarkable replacement is evidently 

 a most important consequence of that curious symbiosis. The two families which 

 produce spongin-fibrilke differ essentially in their relation to the xenophya. These 

 foreign skeletal bodies are enclosed within the maltha alone in the Stannomidae, while in 

 the Spongelidae a part of them, at least, is enclosed in the spongin-fibres. 



Spongin- Skeleton. — The peculiar pure spongin-skeleton, characteristic of the true 

 Keratosa, is found only in two of our deep-sea families, in the Stannomidae (Pis. I. -III.) 



