128 THE HISTORY OF CREATION. 
animals—in Sponges, Medusz, Corals, Worms, Sea-squirts 
Radiated animals, Molluscs, 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 Gastrula in the 
most different animal classes, from Zoophytes up to Ver- 
tebrata, we may, according to the bicgenetic principle, safely 
draw the conclusion that during the Laurentian period there 
existed a common primary form of the six higher anima, 
tribes, which in all essential points was formed like the 
Gastrula, and which we shall call the Gastrza. This Gestrea 
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 wall (which was also the intestinal 
wall) consisted of two layers of cells, 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 
Gastreea 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 Gastreea body has been transmitted to the phase 
of development which directly arises out of the Morula. 
This phase is an oval or round dise consisting of two cell- 
layers or membranes: the outer cell-layer, the animal or 
dermal layer (ectoblast), corresponds to the ectoderm of 
the Gastrea; out of it develops the external, loose skin 
(epidermis), with its glands and appendages, as well as 
the central nervous system. The inner cell-layer, the 
