44 THE VOYAGE OF H.M.S. CHALLENGEE. 



horny skeleton of the former there is not that distinction between stout primary and 

 delicate secondary fibres as in that of the latter. But one may suppose that the stout 

 spongin-fibres of Phyllospongia forming its supporting scaffold have been lost in 

 Psammophyllum and replaced by the chitinous tubes of the symbiotic hydrorhiza. 



Skeleton. — The marked peculiarity of the five deep-sea Spongelidas here described, 

 and the principal distinction between them and the well-known Spongelidse of shallow 

 water, is the complete absence of stout spongin-fibres, forming the firm scaffold of the 

 spongin-skeleton. As already mentioned, these seem to be replaced by the chitinous tubes 

 of the reticular hydrorhiza, produced by the symbiotic Hydroids which traverse the 

 whole body of these curious sponges. The production of the spongin-skeleton, however, 

 is reduced to the scanty and thin fibrillar or lamellae which partly enclose the xenophya, 

 partly connect them. 



Xenophya. — The foreign enclosures, which fill up the scanty transparent maltha of 

 Cerelasma (PI. VI.) and of Psammopihyllum (Pis. IV., V.), and which are partly 

 enclosed by the lamellar or fibrous spongin productions, are in three of the five sjitecies 

 observed Radiolarian tests, in the other two siliceous sponge spicules intermingled with 

 volcanic mineral particles. The calcareous Globigerina ooze, which composes the 

 pseudo-skeleton in most of the Psamminidse and Stannomidae, is rare or entirely absent 

 in these deep-sea Spongelidse, though the bottom at one Station (216) is true Globigerina 

 ooze. The manner in which the xenophya are collected and disposed seems to prove 

 that in these Spongelidaa (as in the Stannomidae) there is a power of selection of 

 materials for the construction of the pseudo-skeleton. The scarce transparent maltha, or 

 the ground-mass of the mesoderm, which surrounds and connects the xenophya, contains 

 two kinds of cells : small stellate, fusiform or roundish connective cells, and amoeboid 

 wandering cells ; the latter probably produce the spongin-skeleton. 



Symbiontes. — The firm scaffold of the body in all the deep-sea Spongelidse is formed 

 not by a network of stout spongin-fibres, as in all the shallow-water inhabitants of this 

 family, but instead by a network of chitinous tubes, which belong to the hydrorhiza 

 of a symbiotic Hydroid. This foreign network traverses all parts of the sponges so 

 densely and continuously (as well in Cerelasma, PI. VI. figs. 2-4, as in 

 Psammophyllum, Pis. IV., V.), that in the preliminary examination I was inclined to 

 regard it as an organ-system of the sponge itself, comparing it with the skeletal 

 network of the Aplysinidse, the branches of which are thin-walled spongin-tubules 

 filled up with a dark medullar mass or pith-substance. But this first supposition was 

 afterwards refuted by the discovery of hydranths, and in some places even of gonangia, 

 being in direct continuity with the chitinous tubes of the network, and filled by the 

 same dark (brown, greenish, or blackish) cellular mass. This mass is evidently the 

 decomposed ccenosarc, the cells of which (entodermal and exodermal) could not be well 

 preserved within the containing dense and decomposing sponge-tissue. The symbiotic 



