Figs. 234 and 235—Prophysema primordiale, a living gastræad. Fig. 234. The whole of the spindle-shaped animal (attached below to the floor of the sea. Fig. 235. The same in longitudinal section. The primitive gut (d) opens above at the primitive mouth (m). Between the ciliated cells (g) are the amœboid ova (e). The skin-layer (h) is encrusted with grains of sand below and sponge-spicules above.
In Prophysema the primitive gut is a simple oval cavity, but in the closely related Gastrophysema it is divided into two chambers by a transverse constriction; the hind and smaller chamber above furnishes the sexual products, the anterior one being for digestion.
Figs. 236–237—Ascula of gastrophysema, attached to the floor of the sea. Fig. 236 external view, 237 longitudinal section. g primitive gut, o primitive mouth, i visceral layer, e cutaneous layer. (Diagram.)
The simplest sponges (Olynthus, Fig. 238) have the same organisation as the Physemaria. The only material difference between them is that in the sponge the thin two-layered body-wall is pierced by numbers of pores. When these are closed they resemble the Physemaria. Possibly the gastræads that we call Physemaria are only olynthi with the pores closed. The Ammoconida, or the simple tubular sand-sponges of the deep-sea (Ammolynthus, etc.), do not differ from the gastræads in any important point when the pores are closed. In my Monograph on the Sponges (with sixty plates) I endeavoured to prove analytically that all the species of this class can be traced phylogenetically to a common stem-form (Calcolynthus).
The lowest form of the Cnidaria is also not far removed from the gastræads. In the interesting common fresh-water polyp (Hydra) the whole body is simply an oval tube with a double wall; only in this case the mouth has a crown of tentacles. Before these develop the hydra resembles an ascula (Figs. 236, 237). Afterwards there are slight histological differentiations in its ectoderm, though the entoderm remains a single stratum of cells. We find the first differentiation of epithelial and stinging cells, or of muscular and neural cells, in the thick ectoderm of the hydra.