THE GASTR.EADA. 1 29 



vegetative or intestinal layer (hypoblast), is oi'iginally the 

 entoderm of the Gastraea ; 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 Amoeba ; (3) the Synamoeba ; (4) the 

 Planaga ; and (5) the Gastrsea. 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 sa)^ 

 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, amongr the tribe of Primaeval animals 

 (Protozoa), which also comprises the still living Infusoria 

 and Gregaringe. 



The phyletic development of the six higher animal tribes, 

 which are all derived from the Gastrsea, deviated at this 

 point in two directions. In other words, the Gastrceach 

 (as we may call the group of forms characterized by the 

 Gastrsea-type of structure), divided into two divergent 

 lines or branches ; the one branch of Gastrgeads gave up 

 free locomotion, adhei'ed to the bottom of the sea, and thus, 

 by adopting an adhesive mode of life, gave rise to the Pro- 

 fdscus, the common primary form of the Animal-'plants 

 (Zoophyta). The other branch of the Gasti'seads retained 

 free locomotion, did not become adherent, and later on 



VOL. ir. K 



