_Disyringa dissimilis_ is remarkable for the perfection of its symmetry, and for the absence of that multiplication of parts which is so common among sponges. It possesses a single inhalant tube and a single osculum (Fig. 107). Until quite recently it stood alone in the restriction of its inhalant apertures to a single area. Kirkpatrick, however, has now described a sponge—_Spongocardium gilchristi_[240]—from Cape Colony, in which the dermal ostia are concentrated in one sieve-like patch at the opposite pole to the single osculum. _Disyringa_ is still without companions in the possession of an inhalant tube. The concentration of ostia into sieve areas occurs again in _Cinachyra_, each sponge possessing in this case several inhalant areas with or without scattered ostia also. ORDER II. LITHISTIDA. The characteristic spicule of Lithistida—the desma—may be a modified calthrop (tetracrepid desma), or it may be produced by the growth of silica over a uniaxial spicule (rhabdocrepid desma) (Fig. 110, _q_), or it may be of the polyaxon type. It is probable that the group is polyphyletic,[241] and that some of its members should remain associated with Tetractinellida, while others should be removed to Monaxonida. Forms with tetracrepid desmas, and those forms with rhabdocrepid desmas which possess triaenes, have Tetractinellid affinities, while forms possessing rhabdocrepid desmas but lacking triaenes, and again those in which the desmas are polyaxon, are probably descendants of Monaxonida. Owing to the consistency of the skeleton Lithistida are frequently found as fossils. The commonest known example is _Siphonia_.[242] As in the case of so many other fossil sponges the skeleton is often replaced by carbonate of lime, a fact which {216}misled some of the earlier investigators but was established by the researches of Sollas and Zittel. SUB-CLASS II. MONAXONIDA.[243]