THE GASTRAADA. I29 
vegetative or intestinal layer (hypoblast), is originally the 
entoderm of the Gastrea; out of it develops the inner 
membrane (epithelium) of the intestinal canal and its glands. 
(Compare my Monograph of the Calcareous Sponges, vol. i. 
p. 466, etc.) 
By ontogeny we have already gained five primordial 
stages of development of the animal kingdom: (1) the 
Moneron; (2) the Amceba; (3) the Synamceba; (4) the 
Planza; and (5) the Gastrea. The former existence of 
these five oldest primary forms, which succeeded one another, 
and which must have lived in the Laurentian period, follows 
as a consequence of the biogenetic principle ; that is to say, 
from the parallelism and the mechanico-causal connection of 
ontogenesis and phylogenesis. (Compare vol. i. p. 309.) In our 
genealogical system of the animal kingdom we may class 
all these animal forms, long since extinct, and, which on 
account of the soft nature of their bodies could leave no 
fossil remains, among the tribe of Primeval animals 
(Protozoa), which also comprises the still living Infusoria 
and Gregarinee. 
The phyletic development of the six higher animal tribes, 
which are all derived from the Gastrea, deviated at this 
point in two directions. In other words, the Gastrwads 
(as we may call the group of forms characterized by the 
Gastreea-type of structure), divided into two divergent 
lines or branches; the one branch of Gastreads gave up 
free locomotion, adhered to the bottom of the sea, and thus, 
by adopting an adhesive mode of life, gave rise to the Pro- 
tascus, the common primary form of the Animal-plants 
(Zoophyta). The other branch of the Gastrzads retained 
free locomotion, did not become adherent and later on 
