[Illustration: FIG. 93.—Scheme to show the arrangement of spicules in the Hexactinellid skeleton. _Canalaria_, microscleres in the walls of the excurrent canals; _Dermalia Autoderm[alia]_, microscleres in the dermal membrane; _D. Hypoderm[alia]_, more deeply situated dermalia; _Dictyonalia_, parenchymalia which become fused to form the skeletal framework of Dictyonina; _Gastralia Autogastr[alia]_, microscleres in the gastral membrane; _Gastralia Hypogastr[alia]_, more deeply situated gastralia; _Parenchymalia Principalia_, main supporting spicules between the chambers; _P. Comitalia_, slender diactine or triactine spicules accompanying the last; _P. Intermedia_, microscleres between the P. principalia; _Prostalia_, projecting spicules; _P. basalia_, rooting spicules, from the base; _P. marginalia_, defensive spicules, round the oscular rim; _P. pleuralia_, defensive spicules, from the sides. (From Delage and Hérouard, after F. E. Schulze.)] The deviations from this ground-plan of Hexactinellid structure are few and simple. They are due to folding of the chamber-layer, or to variations in the shape of the chambers, and to increasing fusion of the spicules to form rigid skeletons. A simple condition of the chamber-layer, like that of the young sponge of Fig. 89, {202}occurs also in some adult Hexactinellids, _e.g._ in _Walteria_ of the Pacific Ocean (Fig. 90). Thus is represented in this order the second type of canal system described among Calcarea. More frequently, however, instead of forming a smooth sheet, the chamber-layer grows out into a number of tubular diverticula, the cavities of which are excurrent canals; these determine a corresponding number of incurrent canals which lie between them. In this way there arises a canal system resembling the third type of Calcarea. By still further pouching so as to give secondary diverticula, opening into the first, a complicated canal system is formed, as, for example, in _Euplectella suberea_.