Fig. 80.—Sycon setosum. Young Sponge. × 200. d, Dermal cell; g, gastral cell; o, osculum; p, pore cell; sp1, monaxon; sp3, triradiate spicule. (After Maas.)
The walls of the paragaster are known as the "gastral cortex"; they contain quadriradiate spicules, of which the triradiate systems lie tangentially in the gastral cortex, while the apical ray projects into the paragaster, and is no doubt defensive. The distal ends of the chambers bristle with tufts of oxeate spicules, and the separate chambers are distinguishable in surface view. It is interesting to notice that in some species of Sycon, the gaps between the distal ends of the chambers are covered over by a delicate perforated membrane, thus leading on, as we shall see presently, to the next stage of advance.[[219]] The larva of Sycon is an amphiblastula (see p. [227]). Fig. 80 is a drawing of the young sponge soon after fixation; it would pass equally well for an ideally simple Ascon or, neglecting the arrangement of the spicules, for an isolated radial tube of Sycon. Figs. 81, 82 show the same sponge, somewhat older. From them it is seen that the Sycon type is produced from the young individual, in what may be called its Ascon stage, by a process of outgrowth of tubes from its walls, followed by restriction of choanocytes to the flagellated chambers. Minute observation has shown[[220]] that this latter event is brought about by immigration of pinacocytes from the exterior. These cells creep through the jelly of the dermal layer and line the paragaster as fast as its original covering of choanocytes retreats into the newly formed chambers.
Fig. 81.—S. setosum. Young Sponge, with one whorl of radial tubes. o, Osculum; p, pore; sp1, monaxon; sp4, 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 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.
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; sp4, tetraradiate spicule; T, radial tube. (After Maas.)
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.)