60 THE RIDDLE OF THE UNIVERSE. 



sea- squirt, or ascidian. Even in some of the worms, 

 the radiata and the articulata, a similar formation of 

 the germinal layers was pointed out by the same 

 observer. I myself was then (since 1886) occupied 

 with the embryology of the sponges, corals, medusae, 

 and siphonophora, and, as I found the same formation 

 of two primary germ-layers everywhere in these lowest 

 classes of multicellular animals, I came to the conclu- 

 sion that this important embiyonic feature is common 

 to the entire animal world. The circumstance that 

 in the sponges and the cnidaria (polyps, medusae, 

 etc.) the body consists for a long time, sometimes 

 throughout life, merely of two simple layers of 

 cells, seemed to me especially significant. Huxley 

 had already (1849) compared these, in the case of the 

 medusae, with the two primary germinal layers of the 

 vertebrates. On the ground of these observations and 

 comparisons I then, in 1872, in my Philosojihy of the 

 Calcispongice, published the " theory of the gastraea," 

 of which the following are the essential points : — 



I. — The whole animal world falls into two essen- 

 tially different groups, the unicellular primitive 

 animals (Protozoa) and the multicellular animals 

 with complex tissues (Metazoa). The entire organism 

 of the protozoon (the rhizopods or the infusoria) 

 remains throughout life a single simple cell (or 

 occasionally a loose colony of cells without the 

 formation of tissue, a camobium). The organism of 

 the metazoon, on the contrary, is only unicellular at 

 the commencement, and is subsequently built up of a 

 number of cells which form tissues. 



II. — Hence the method of reproduction and develop- 

 ment is very different in each of these great categories 

 of animals. The protozoa usually multiply by non- 



