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).
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).
All the Amphidiscophora have Lyssacine skeletons; in the Hexasterophora both types of skeleton occur. The subdivision of the Hexasterophora is determined by the presence or absence of uncinate spicules. An "uncinatum" is a diactine spicule, pointed at both ends and bearing barbs all directed towards one end. This method of classification gives us a wholly Dictyonine order, Uncinataria, and an order consisting partly of Dictyonine, partly of Lyssacine genera, which may be distinguished as the Anuncinataria. Ova have rarely been found, and sexually produced larvae never; but Ijima has found archaeocyte clusters in abundance, and his evidence is in favour of the view that they give rise asexually to larvae, described by him in this class for the first time (see p. [231]).
Both sub-classes are represented in British waters: the Amphidiscophora by Hyalonema thomsoni and Pheronema carpenteri; the Hexasterophora by Euplectella suberea and Asconema setubalense, and of course possibly by others.
Hyalonema thomsoni, one of the glass-rope sponges, was dredged by the Porcupine off the Shetland Islands in water of about 550 fathoms. The spindle-shaped body of the sponge is shown in Fig. 97. Its long rooting tuft is continued right up its axis, to end in a conical projection, which is surrounded by four apertures leading into corresponding compartments of the paragaster.
Fig. 97.—Hyalonema thomsoni. A, Whole specimen with rooting tuft and Epizoanthus crust; B, pinulus, a spicule characteristic of but not peculiar to the Amphidiscophora, occurring in the dermal and gastral membranes; C, amphidisc with axial cross; D, distal end of rooting spicule with grapnel. (After F. E. Schulze.)
The crust of Anthozoa of the genus Epizoanthus (p. [406]) on the rooting tuft is a constant feature in this as in other species of Hyalonema. It contributed to make the sponge a puzzle, which long defied interpretation. The earliest diagnosis the genus received was the "Glass Plant." Then the root tuft was thought to be part of the Epizoanthus, which was termed a "most aberrant Alcyonarian with its base inserted in a sponge"; next we hear of the sponge as parasitic on the Sea Anemone. Finally, the root tuft was shown to be proper to the sponge, which was, however, figured upside down, till some Japanese collectors described the natural position, or that in which they were accustomed to find it.