_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 _Pagurus_, 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 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.[252] 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. [Illustration: FIG. 108.—A, calcareous corpuscle detached by _Cliona_; B, view of the galleries excavated by the Sponge. (After Topsent.)] _Ficulina ficus_ 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 metamps, one of which, from its fig-like shape, gives it its name. {220}SUB-CLASS III. CERATOSA. The Ceratosa are an assemblage of ultimate twigs shorn from the branches of the Monaxonid tree. They are therefore related forms, but many of them are more closely connected with their Monaxonid relatives than with their associates in their own sub-class. The genera _Aulena_ and _Phoriospongia_, placed by v. Lendenfeld among Ceratosa, by Minchin among Monaxonida, show each in its own way how close is the link between these two sub-classes. _Aulena_ possesses in its deeper parts a skeleton of areniferous spongin fibres, in fact a typical Ceratose skeleton; but this is continuous with a skeleton in the more superficial parts, which is composed of spongin fibres echinated by spicules proper to the sponge, and precisely comparable to the ectyonine fibres of some Monaxonida. _Phoriospongia_, as far as its main skeleton is concerned, is a typical Ceratose sponge, with fibres of the areniferous type, but it possesses sigmata free in the flesh. The sub-class is confined to shallow water, no horny sponge having been dredged from depths greater than 410 fathoms.[253] The greatest number occur at depths between 10 and 26 fathoms.