II INTRODUCTION TO STUDY OF METAZOA 57 



(blastula) arrange themselves in all Metazoa in a double layer, and in 

 many cases they do this in the simplest manner, i.e. by the wall of the 

 hollow sphere sinking in at one spot. The sunken portion lines the 

 non-invaginated portion of the hollow sphere, and so we have a 

 pouch with a double wall (ectoderm and endoderm). The inner 

 layer, the endoderm, surrounds a cavity, the areh-enteron, which 

 opens outward by an aperture, the primitive mouth or blastopore. 

 The outer layer, or ectoderm, everywhere supplies the outer integu- 

 ment and the nervous system of the animal ; the inner, the enteric 

 epithelium and the glands which proceed from it. This germinal form 

 is called Gastrula. 



2. The body of one of the lowest Metazoa, e.g. one of the simplest 

 Ccelenterata, throughout life consists of two layers, which in all essentials 

 correspond with the two germinal layers of the Gastrula. The outer 

 layer, the ectoderm, represents the outer integument ; the inner, the 

 endoderm, the epithelial wall of the intestinal cavity. The latter 

 surrounds the intestinal cavity, which opens at one point, the mouth. 



From these two series of facts the following conclusions may be 

 drawn : — ■ 



1. All Metazoa are descended from one common ancestral form, 

 which possessed essentially the structure of one of the lower Gcdenterata. 

 This hypothetical ancestor, the Gastrcea, is met with in all Metazoa 

 as a transitionary stage in their developmenl^^as a Gastrula. 



2. The Gastrcea itself arose in a similar way from a Protozoan 

 colony in the shape of a hollow sphere by the formation and the 

 gradual deepening of a depression, just as in the individual develop- 

 ment of many animals the gastrula arises by invagination out of a 

 hollow group of cells, the product of the segmentation of the egg. 



The three chief Divisions of the Metazoa. 



A. The body consists essentially of two layers — the ectoderm and the endoderm. 

 There is no middle layer as a rule, and where such does occur, its close relation to 

 either the ectoderm or the endoderm, or to both, is clear. Intestine with one external 

 aperture — the mouth. A body cavity between the intestine and the integument is 

 wanting ; so also are blood-vessels and excretory organs. A nervous system is either 

 wanting, or, where it occurs, is little centralised. 



Comprises : The Second Race or Phyliun of the Animal Kingdom — Zoophyta or 

 Ocelenterata. 



B. With well-developed mesoderm sharply distinguished from the ectoderm and 

 endoderm. Gastric cavity with a single aperture opening externally (mouth). Body 

 cavity and blood- vascular system wanting. Excretory organs (water- vascular system) 

 present. Nervous system centralised. 



Comprises : The Third Race or Phylum of the Animal Kingdom— Pfoto&s. 



C. With well developed mesoderm, sharply distinguished from the ectoderm and 

 the endoderm. Intestine vsdth two external apertures (oral and anal). Generally 

 with a, body cavity in mesoderm. Blood-vascular and excretory systems usually 

 present. Nervous system centralised. 



