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. [Illustration: 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. {203}96). The former set of genera constitute the sub-class Amphidiscophora, the latter the Hexasterophora. [Illustration: FIG. 95.—Portion of body-wall of _Hyalonema_, in section, showing the irregular chambers.] SUB-CLASS 1. AMPHIDISCOPHORA.—_Amphidiscs are present, hexasters absent. A tuft of rooting spicules or basalia is always present. The ciliated chambers deviate more or less from the typical thimble shape, and the membrana reticularis is continuous from chamber to chamber_ (Figs. 94, 95, 97). [Illustration: FIG. 96.—Hexasters. A, Graphiohexaster; B, floricome; C, onychaster.] SUB-CLASS 2. HEXASTEROPHORA.—_Hexasters are present, amphidiscs absent. The chambers have the typical regular form, and are sharply marked off from one another_ (Figs. 90, 96).