THE DISCOVERER 75 



brace only the one-celled ; and the Coelenterata and 

 Coelomata, grouped as many-celled, under the term 

 Metazoa. Even this scheme is under modification — 

 the animals known as sponges being now assigned a 

 separate place as " an independent and sterile branch 

 of the tree of life," a branch, perhaps, in direct 

 descent from the one-celled organism. 1 The Me- 

 dusae, hydra, and sea-anemones are grouped under 

 Coelenterata, or hollow-bodied, comprising all two- 

 layered animals. The Coelomata comprise all 

 animals in which a third foundation-membrane has 

 been developed, and which possess a coelom, or true 

 stomach, with blood-vessels. They embrace every 

 animal from a worm to a man. 



Huxley's next step was to compare the two founda- 

 tion-membranes of the Coelenterata with the serous 

 and mucous layers of the embryos of vertebrates ; and 

 here, although he then guessed it not, he made a con- 

 tribution of the highest importance to the doctrine of 

 descent. Von Baer had shown the resemblances be- 

 tween all back-boned animals in their passage from 

 the embryo to the adult state, and Huxley showed 

 that, in still earlier stages of their development, they 

 exhibited the two foundation-membranes of the 

 Coelenterata, thus recording, as it were, the history 



1 A Treatise on Zoology. Part II. The Porifera and Coelen- 

 terata. Edited by E. Ray Lankester, F.R.S. 



