The various spicules are named, irrespective of their form, according to their position and corresponding function. The arrangement of the spicules is best realised by means of a diagram (Fig. 93).
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, 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.
To return to the skeleton, the most complete fusion is attained by the deposit of a continuous sheath of silica round the apposed parallel rays of neighbouring spicules. This may be termed the dictyonine type of union, for it occurs in all those forms originally included under the term Dictyonina, in which the cement is deposited pari passu with the formation of the spicules. In other cases connecting bridges of silica unite the spicules, or there may be a connecting reticulum of siliceous threads, or, again, rays crossing obliquely may be soldered together at the point of contact. These more irregular methods occur in species where the spicules are free at their first formation. Spicules originally free may later be united in a true Dictyonine fashion. The terms Lyssacina and Dictyonina are useful to denote respectively: the former all those Hexactinellida in which the spicules are free at their first formation, and the latter those in which the deposit of the cementing layer goes hand in hand with the formation of the spicules. But the terms do not indicate separateness of origin of the groups denoted by them, for there is evidence that Dictyonine types have been derived repeatedly from Lyssacine types, and that in fact every Dictyonine was once a Lyssacine.
Fig. 94.—Amphidisc, at a are traces of the four missing rays.
The real or natural cleft in the class lies between those genera possessing amphidiscs (Figs. 94, 97) among their microscleres, and all the remainder of the Hexactinellida which bear hexasters (Fig. 96). The former set of genera constitute the sub-class Amphidiscophora, the latter the Hexasterophora.
Fig. 95.—Portion of body-wall of Hyalonema, in section, showing the irregular chambers.