138 THE HISTORY OF CREATION, 
as in Sea-nettles, proceeds from the Gastrula. (Compare the 
Ascula of the calcareous sponge on the Frontispiece, Fig 7, 8.) 
For after the Gastrula of zoophytes has for a time swum 
about in the water it sinks to the bottom, and there adheres 
by that pole of its axis which is opposite to the opening of 
the mouth. The external cells of the ectoderm draw in 
their vibrating, ciliary hairs, whereas, on the contrary, the 
inner cells of the entoderm begin to form them. Thus the 
Ascula, as we call this changed form of larva, is a simple 
sack, its cavity (the cavity of the stomach or intestine) 
opening by a mouth externally, at the upper pole of the 
longitudinal axis (opposite the basal point of fixture). The 
entire body is here in a certain sense a mere stomach or 
intestinal canal, as in the case of the Gastrula. The wall of 
the sack, which is both body wall and intestinal wall, con- 
sists of two layers or coats of cells, a fringed entoderm, 
or gastral layer (corresponding with the inner or vegeta- 
tive germ-layer of the higher animals), and an unfringed 
exoderm or dermal layer (corresponding with the external 
or animal germ-layer of the higher animals). The original 
Protascus, a true likeness of which is still furnished by 
the Ascula, probably formed egg-cells and sperm-cells out 
of its gastral layer. 
The Protascads—as we will eal the most ancient group 
of vegetable animals, represented by the Protascus-type— 
divided into two lines or branches, the Spongiz and the 
Sea-nettles, or Acalephz. I have shown in my Monograph 
of the Calcareous Sponges (vol. i. p. 485) how closely these 
two main classes of Zoophytes are related, and how they 
must both be derived, as two diverging forms, from the 
Protascus-form. ‘The primary form of Spongize, which I 
