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The Sponges are sometimes placed by themselves as a separate phylum, the Porifera. With the exception of a few freshwater genera, they are of marine habit and to be found at all depths between low tide (littoral) and deep water (abyssal). Sponges are either fixed or lie loosely on the sea-floor. They possess no organs of locomotion, and have no distinct axis or lateral appendages. They exist by setting up currents in the water whereby the latter is circulated through the system, carrying with it numerous food particles, their tissues being at the same time oxygenated. Their framework, in the siliceous and calcareous sponges, is strengthened by a mineral skeleton, wholly or partially capable of preservation as a fossil.
Cambrian and Ordovician Sponges.—
The oldest rocks in Australia containing the remains of Sponges are the Cambrian limestones of South Australia, at Ardrossan and elsewhere. Some of these sponge-remains are referred to the genus Protospongia, a member of the Hexactinellid group having 6-rayed skeletal elements. When complete, the Protospongia has a cup- or funnel-shaped body, composed of large and small modified spicules, which form quadrate areas, often seen in isolated or aggregated patches on the weathered surface of the rock. Protospongia also occurs in the Lower Ordovician slates and shales of Lancefield (P. oblonga), and Bendigo (P. reticulata and P. cruciformis), in Victoria ([Fig. 67 A]). At St. David’s, in South Wales, the genus is found in rocks of Middle Cambrian age. The South Australian limestones in which Protospongia occurs are usually placed in the Lower Cambrian.
Fig. 67—PALAEOZOIC SPONGES, &c.
A—Protospongia reticulata, T. S. Hall. Low. Ordovician. Bendigo.
B—Receptaculites fergusoni, Chapm. Silurian. Wombat Creek, Vict.
C—R. australis, Salter. (Section of wall, etched, after Eth. & Dun) Mid. Devonian. Co. Murray, N.S.W.
D—Protopharetra scoulari, Eth. fil. Cambrian. S.A.
Another genus of Sponges, Hyalostelia, whose affinities are not very clear, occurs in the South Australian Cambrian at Curramulka. This type is represented by the long, slightly bent, rod-like spicules of the root-tuft, and the skeletal spicules with six rays, one of which is much elongated.
Stephanella maccoyi is a Monactinellid sponge, found in the Lower Ordovician (Bendigo Series) of Bendigo, Victoria.
Silurian Sponges.—