1 84 Elementary Zoology. 



(endoderm) being different from those of the outer layer 

 (ectoderm). 



That Hydra is a simple form of metazoon, in which these 

 characters are not much more than just complied with, will be 

 obvious from the preceding description. It is a two-layered 

 sac, only complicated by the outgrowth of the tentacles round 

 the orifice at the anterior end. 



That this definition also applies to the remainder of the 

 animals treated of in this volume will be clearer from their 

 development. In all of them the embryo is at one time clearly 

 composed of a two-layered sac, the gastrula, with a central 

 cavity and an aperture at one end of the body. Disguised 

 though this gastrula stage may be, owing to various reasons, 

 but chiefly the mechanical effect of large masses of yolk in the 

 egg, the gastrula stage has been identified in all animals whose- 

 development has been studied. They all pass through a two- 

 layered stage, in which they more or less closely resemble 

 Hyd7-a. A second essential character of the Metazoa may be 

 stated as follows : — 



2. The cells of the metazoon body are grouped into tissues 

 which act as a whole,, not each cell for itself, and in subordina- 

 tion to the needs of the individual. 



This character is placed second in place and in importance 

 to the first character of the Metazoa, and for the following 

 reasons : — 



In the first place all the tissues of the body are not a group 

 or layer of cells which act as a whole and in subordination to 

 the rest of the body. The reproductive tissues are in some 

 respects an exception. The ovum of the hydra, as already 

 said, actually devours the neighbouring cells precisely as if it 

 were a parasitic Amxba. If it be objected to this that this is 

 really an act for the common good of the tissues composing 

 the body, for the organism as a whole, as it is essential for the 

 propagation of the species, it may be replied that the protozoa 

 show precisely the same phenomenon, i.e. the reproductive 

 cells of Volvox and Proterospongia. In the same Protozoa and in 

 other multicellular forms it is difficult to suppose that the cells 

 farthest from the surface of the colony are not fed by particles 



