[Illustration: FIG. 81.—_S. setosum._ Young Sponge, with one whorl of radial tubes. _o_, Osculum; _p_, pore; _sp__{1}, monaxon; _sp__{4}, quadriradiate spicule. (After Maas.)] With a canal system precisely similar to that of _Sycon_, _Ute_ (Fig. 83) shows an advance in structure in the thickening of the dermal layers over the distal ends of the chambers. The dermal thickenings above neighbouring chambers extend laterally and {190}meet; and there results a sheet of dermal tissue perforated by dermal ostia, which open into the inhalant canals, and strengthened by stout spicules running longitudinally. This layer is termed a cortex; it covers the whole sponge, compacting the radial tubes so that they form, together with the cortex, a secondary wall to the sponge, which is once more a simple sac, but with a complex wall. The cortex may be enormously developed, so as to form more than half the thickness of the wall (Fig. 84). The chambers taken together are spoken of as the chamber layer. [Illustration: FIG. 82.—_Sycon raphanus._ A, Longitudinal section of young decalcified Sponge at a stage somewhat later than that shown in Fig. 81. B, Transverse section of the same through a whorl of tubes. _d_, Dermal membrane; _g_, gastral membrane; _H_, paragaster; _sp__{4}, tetraradiate spicule; _T_, radial tube. (After Maas.)] [Illustration: FIG. 83.—Transverse section of the body-wall of _Ute_, passing longitudinally through two chambers. _a.p_, Apopyle; _d.o_, dermal ostium; _fl.ch_, flagellated chamber or radial tube; _i.c_, inhalant canal; _p_, prosopyle. (After Dendy.)] We have already alluded to the resemblance between a young Ascon person and a radial tube of _Sycon_—a comparison which calls to mind the somewhat strange view of certain earlier authors, that the flagellated chambers are really the sponge individuals. If now we suppose each Ascon-like radial tube of _Sycon_ to undergo that same process of growth by which the {191}_Sycon_ itself was derived from the Ascon, we shall then have a sponge with a canal system of the type seen in _Leucandra_ among British forms, but more diagrammatically shown in the foreign genus _Leucilla_ (Fig. 85). The foregoing remarks do not pretend to give an account of the transition from _Sycon_ to _Leucilla_ as it occurred in phylogeny. For some indication of this we must await embryological research. In _Leucandra_ the fundamental structure is obscured by the irregularity of its canal system. It shows a further and most important difference from _Leucilla_ in the smaller size and rounded form of its chambers. This change of form marks an advance in efficiency; for now the flagella converge to a centre, so that they all act on the same drop of water, while in the tubular chamber their action is more widely distributed and proportionately less intense (see p. 236).