The same Spongicola lives in pairs in Hyalonema sieboldi. Another case of apparently constant association is that of the Hydroid stocks which inhabit Walteria. F. E. Schulze describes Stephanoscyphus mirabilis (see p. [318]) in a specimen of Walteria flemmingi; the presence of the polyp causes the sponge to grow out into little dome-shaped elevations, each of which shelters one polyp; while in W. leuckarti Ijima finds a similar association in every specimen examined.
Fossil Hexactinellida.
This group has the distinction of including among its Lyssacine members the oldest known sponge, Protospongia fenestrata, of Cambrian age (Salter). As preserved it consists of a single layer of quadriradiate, or possibly quinqueradiate spicules, which, arranged as a square meshed lattice, supported the superficial layer of the sponge (Fig. 101). Whether or not the fossil represents the whole of the sponge-skeleton does not appear.[[236]]
Fig. 101.—Part of the specimen of Protospongia fenestrata in the Sedgwick Museum, Cambridge. Nat. size. (After Sollas.)
Fig. 102.—A portion of the outer surface of a Receptaculitid, Acanthoconia barrandei, in which the expanded outer rays of the spicules are partially destroyed, revealing the four tangential rays beneath, × 3. (After Hinde.)
The extraordinary Receptaculitidae are probably early Lyssacine forms: they are cup- or saucer-shaped fossils, abundant in Silurian and above all in Devonian strata, and have been "assigned in turn to pine cones, Foraminifera, Sponges, Corals, Cystideans," and Tunicata. Hinde[[237]] brings forward important arguments for retaining them among Hexactinellida. The only elements in the skeleton of the simpler genera, e.g. Ischadites, are structures comparable to Hexactinellid spicules. The surface of the fossil presents a series of lozenges forming a regular mosaic. Each lozenge is the expanded end of one of the rays of a spicule; it conceals four rays in one plane, tangential to the wall of the cup-shaped fossil, while the sixth ray projects vertically to the wall into the cavity of the cup. In the genus Receptaculites itself there is an inner layer of plates abutting against the inner ends of the sixth rays, and at present problematic. An axial canal is present in each of the rays—the six canals meeting at the centre of the spicule. Special chinks between the spicules appear to have provided a passage for the water current.
The beautiful Ventriculites, so common in the Chalk and present in the Cambridge Greensand, are historically interesting, for the fact that they are fossil Hexactinellida of which the general and skeletal characters were very minutely described by Toulmin Smith long before recent representatives of the group were known. In common with a number of fossil Dictyonine species they are distinguished by the perforation of the nodes, a character due to the fact that the siliceous investment which unites the spicules together stops short before reaching the centre of each spicule, and bridges across the rays so as to form a skeleton octahedron. This character is rare in recent Hexactinellids, but, as first pointed out by Carter, it is presented by one or two forms, of which Aulocystis grayi Bwk is best known. The majority of the fossil Hexactinellida belong to the Dictyonine section, a fact attributable to the greater coherence of their skeleton. The "Dictyonina" are to be reckoned among the rock-builders of Jurassic and Cretaceous times.
