. The Cambridge natural history. Zoology. MONAXONIDA 219 been calculated^ that sponge skeletons may give rise with considerable rapidity to beds of flint nodules; in fact, it appears that a period so short as fifty years is sufficient for the formation of a bed of flints out of the skeletons of sponges alone. Suberites domuncula is well known for its constant symbiosis with the Hermit crab. The young sponge settles on a Whelk or other shell inhabited by a Fagurus, and gradually envelops it, becoming very massive, and completely concealing the shell, without however closing its mouth. The apert
. The Cambridge natural history. Zoology. MONAXONIDA 219 been calculated^ that sponge skeletons may give rise with considerable rapidity to beds of flint nodules; in fact, it appears that a period so short as fifty years is sufficient for the formation of a bed of flints out of the skeletons of sponges alone. Suberites domuncula is well known for its constant symbiosis with the Hermit crab. The young sponge settles on a Whelk or other shell inhabited by a Fagurus, and gradually envelops it, becoming very massive, and completely concealing the shell, without however closing its mouth. The aperture of this always remains open to the exterior, however great the growth of the sponge, a tubular passage being left in front of it, which. Fio. 108.—A, calcareous corpuscle detached by Cliona ; E, view of the galleries excavated by the Sponge. (After Topsent.) continues the lumen of the shell and maintains its spiral direction. When the crab has grown too big for the shell, it merely advances a little down this passage. The shell is never absorbed, as was once supposed.^ The crab, besides being provided with a continually growing house, and being thus spared the great dangers attending a shift of lodgings, benefits continually by the concealment and protection afforded by the massive sponge; the latter in return is conveyed to new places by the crab. Ficulina Jims is sometimes, like >S^. domuncula, found in symbiosis with Fagurus, but the constancy of the association is wanting in this case. The sponge has several metamps, one of which, from its fig-like shape, gives it its name. 1 Sollas, Challenger Monograijh, xxv. pt. Ixiii. 1888, p. Ixxxix. 2 Topsent, Arch. Zool. Exp. (3) viii. 1900, p. 226. For an account of certain very remarkable structures termed diaphragms in Cliona mucronata and C. ensifera, see Sollas, Ann. Mag. Nat. Hist. (5) i. 1878, p. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readabi
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