138 THE HISTORY OF CREATIOiy. 



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-cella out 

 of its gastral layer. 



The Protascads — as we will call the most ancient group 

 of vegetable animals, represented by the Protascus-type — 

 divided into two lines or branches, the Spongias and the 

 Sea-nettles, or Acalephse. 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 Spongise, which I 



