MONAXONIDA 219 



been calculated^ that sponge skeletons may give rise with 

 considerable rapidity to beds of flint nodules ; in fact, it 

 appears tliat a period so short as fifty years is sufficient for 

 the formation of a bed of flints out of the skeletons of sponges 

 alone. 



Suherites 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 tliis 

 always remains open to the exterior, however great the growth 

 of the sponge, a tubular passage being left in front of it, which 



Fig. 108. — A, calcareous corpuscle detached by Cliona ; B, 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. 



FiciLlina Jicus is sometimes, like S. domuncula, found in 

 symbiosis with Pagurus, but the constancy of the association is 

 wanting in this case. The sponge has several metaraps, one of 

 which, from its fig-like shape, gives it its name. 



' Sollas, Challenger Monograi^h, xxv. pt. Ixiii. ISSS, p. Ixxxix. 



^ Topsent, Arch. Zool. Exp. (3) viii. 1900, p. 226. For an account of certain 

 very remarkable structures termed diaphragms in Cliona mucronata and C. cnsifera, 

 see Sollas, Ami. Mag. Nat. Hist. (5) i. 1878, p. 54. 



