20 



THE VOYAGE OF H.M.S. CHALLENGER. 



remains of other organisms which live in the same depths may also be taken up by the 

 growing sponge, as, for instance, the siliceous spicules of Hyalospongise, and the 

 calcareous fragments of echinoderms and other lower animals. Among the twenty-six 

 Deep-sea Keratosa here described, ten species possess a siliceous skeleton composed of 

 Eadiolarian shells, eight species a calcareous skeleton composed of Foraminifera shells, 

 three species a mineral skeleton, composed of the volcanic particles of red clay, and five 

 species a mixed skeleton, composed of the various elements of the three kinds of ooze 

 and also of various sponge spicules. The disposition of the different skeletal elements in 

 the four families of Deep-sea Keratosa will be seen from the following table : — 



The xenophya fill up the whole maltha in all the Deep-sea Keratosa, and are 

 immediately enclosed by the clear transparent ground-mass of the mesoderm. The 

 family Spongelidse differs from the other three in having a portion of the xenophya 

 also connected and partly enclosed by the skeletal fibres ; in Psammophyttum (Pis. IV., 

 V.) the enclosure by the spongin -fibres is similar to that in the common Spongelidae 

 (Spongelia, Dysidea, &c), but in Cerelasma each single xenophyum is surrounded by a 

 capsular spongin-envelope, and these are connected by branched spongin-lamellse, which 

 form a reticular scaffold (PL VI.). The new family Stannomidae differs from all other 

 Keratosa in the peculiar fact, that the whole mesoderm is traversed by innumerable 

 bundles of fine spongin-fibrillse, but these run between the xenophya and never enclose 

 them (Pis. I.-III.). 



Chitinous Tubes of Symbiotic Hydroids. 



One of the most remarkable features of our Deep-sea Keratosa, characteristic of the 

 majority of them, is their symbiosis with certain Hydroids. The cylindrical, branched, 

 and anastomosing chitinous tubes, which compose the reticular hydrorhiza of the latter, 



