The porifera and coelentera . annomidae, and it is very probable that they are not sponges at all(see p. 154). There remain for mention, finally, the peculiar filaments found incertain genera (Hircinia, Utelot-pongiis, etc.), combined with a sponginskeleton of the ordinary type. Each filament is a long slender twistedthread, slightly thicker iu the middle than towards the extremities, andterminating at each end in a knob. The form has been aptly coni-j)ared to that of an ordinary skijiping-rope, with pear-shaped filament has a thin sheath enclosing a softer medulla, traversedfrom
The porifera and coelentera . annomidae, and it is very probable that they are not sponges at all(see p. 154). There remain for mention, finally, the peculiar filaments found incertain genera (Hircinia, Utelot-pongiis, etc.), combined with a sponginskeleton of the ordinary type. Each filament is a long slender twistedthread, slightly thicker iu the middle than towards the extremities, andterminating at each end in a knob. The form has been aptly coni-j)ared to that of an ordinary skijiping-rope, with pear-shaped filament has a thin sheath enclosing a softer medulla, traversedfrom end to end by an axial thread. The greatest uncertainty prevails SPONGES •43 as to the true nature of these structures. Their chemical nature hasbeen shown to be different from that of spongin (Schulze) ; but whilesome authors are inclined to regard them as foreign to the sponge, andprobably organisms of a symbiotic or parasitic nature, others considerthem as true products of the sponge tissues. Haeckel, amongst the latter,. Fiu. .14. SpoiiRin fibres of Spongelia avara, loaded with foreign , connecting fibre. (After F. E. Schulze.) ), principal tibrc compares them with the fibrillae of Stannomidae, while Fol professes totrace their origin to fusiform cells of the connective tissue layer, andconsiders that the family Filiferae (0. Schmidt) should be reinstated forthe horny sponges characterised by the possession of filaments. Loistdsuggests that they are intracellular spongin filaments of the same nature 144 SPONGES as the elastic fibrillae described by him in Reniera. The question cannotat present be decided. Phylofjeinj of Kcratosc Skeletons.—In dealing with the Mouaxonida, theevolution of the pure spongiii fibre, by gradual increase of the spongin andatrophy of the spicules in the skeletal fibres of that group, has alreadybeen traced (see above, p. 139). It is highly probable not only that mostKeratose skeletons have so originated, but that the evolut
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