128 THE HISTORY OF CREATION. 



animals — in Sponges, Medusae, Corals, Worms, Sea-squirts, 

 Radiated animals, MoUuscs, and even in the lowest Ver- 

 tebrata (Amphioxus : compare p. 200, Plate XII., Fig. B 4 ; 

 see also in the same place the Ascidian, Fig. A 4). 



From the ontogenetic occurrence of the Grastrula in the 

 most different animal classes, from Zoophytes up to Ver- 

 tebrata, we may, according to the biogenetic principle, safely 

 draw the conclusion that during the Lanrentian period there 

 existed a common primary form of the six higher animal 

 tribes, which in aU essential points was formed like the 

 Gastrula, and which we shall caU the Gastrsea. This Gastrasa 

 possessed a perfectly simple globular or oval body, which 

 enclosed a simple cavity of like form, namely, the progaster ; 

 at one of the poles of the longitudinal axis the primary 

 intestine opened by a mouth which served for the reception 

 of nutrition. The body waU (which was also the intestinal 

 waU) consisted of two layers of ceUs, the unfringed entoderm, 

 or intestinal layer, and the fringed ectoderm, or skin-layer ; 

 by the motion of the cilia or fringes of the latter the 

 Gastrsea swam about freely in the Laurentian ocean. Even 

 in those higher animals, in the ontogenesis of which the 

 original Gastrula form has disappeared, according to the laws 

 of abbreviated inheritance (vol. i. p. 212), the composition 

 of the Gastrsea body has been transmitted to the phase 

 of development which directly arises out of the Morula. 

 This phase is an oval or round disc consisting of two cell- 

 layers or membranes : the outer ceU-layer, the animal or 

 dermal layer (ectoblast), corresponds to the ectoderm of 

 the Gastrsea ; out of it develops the external, loose skin 

 (epidermis), with its glands and appendages, as weU as 

 the central nervous system. The inner cell-layer, the 



