Fig. 32.—Spicules of Lithistid sponge (Trichospongia of Billings). From the Cambrian of Labrador.
Many years ago the keen eye of the late lamented Salter detected in a stain on the surface of a slab of Cambrian slate the remains of a sponge; and minute examination showed that its spicules crossed each other, and formed lattice-work on the hexactinellid plan. Salter boldly named it Protospongia (the first sponge), and it is still the earliest that we know (Fig. 30). Thus the type whose skeleton is the most perfect in a mechanical point of view takes the lead. It is continued in the Silurian in many curious forms, of which the stalkless sponges (Astylospongia) are the most common ([Fig. 31]). It perhaps attains its maximum in the Cretaceous, from which the beautiful example in [Fig. 29] is taken, and it still flourishes, giving us the most beautiful of all recent forms. Before the close of the Cambrian there were other sponges of the Lithistid type. [Fig. 32] represents a group of spicules from the Calciferous (Lowest Silurian or Upper Cambrian) of Mingan,[10] and which probably belong to a large Lithistid sponge of that early time. The Lithistids have been recognised in the Upper Silurian and Carboniferous, and continuing upward to the Cretaceous, there become vastly numerous, while their modern representatives are by no means few. The silicious sponges with simple spicules appear to have existed as far back as the Siluro-Cambrian, and there is believed to be almost as early evidence of horny or corneous sponges. The calcareous sponges have been recognised as far back as the Silurian.[11] Thus from the close of the Palæozoic all the types of sponges seem to have existed side by side; and in the Cretaceous period, when such large areas of our continents were deeply submerged, they attained a wonderful development, perhaps not equalled in any other era of the earth’s history.
| Fig. 33.—Oldhamia antiqua (Forbes). | Fig. 34.—Dictyonema sociale. Enlarged. Lingula flags.—After Salter. |
Sponges may be regarded as the highest or most complex of the Protozoa or the lowest of the Coelenterates. We have no links wherewith to connect them with the lower Protozoa of the Eozoic period; and through their long history, though very numerous in genera and species, they show no closer relationship with the Foraminifera below, and the Corals above, than do their successors in the modern seas. They thus stand very much apart; and modern studies of their development and minute structures do not seem to remove them from this isolation. Though we are treating here of inhabitants of the sea, it may be proper to mention that Geinitz has described two species from the Permian which he believed to be early precursors of the Spongillæ, or fresh-water sponges; but more recently he seems to regard them as probably Algæ. Young has, however, recently found true spicules of Spongilla in the Purbeck beds.[12]
Fig. 35.—Dictyonema Websteri (Dn). Niagara formation.
a, Enlarged portion (Acadian Geology).
