THE EVOLUTION OF INVERTEBRATES 297 



for example, as morphology shows, is a very simple form com- 

 pared with the earthworm. Not only has Hydra fewer organs 

 than the earthworm, but reference to the study of develop- 

 ment of the latter will show that the two layers of cells 

 which constitute the entire animal Hydra represent a very 

 early stage in the development of the earthworm. 



There is an additional point of value to be observed in 

 the comparison of Hydra and the earthworm. There are two 

 phyla of animals that in development have two germ-layers, 

 Porifera and Coelentera. All the others (except the Protozoa) 

 have three germ-layers. The third germ-layer, the mesoderm, 

 offers a beginning place for organs not developed in the 

 ectoderm and endoderm, and groups of animals possessing it 

 develop more kinds of organs than those which do not have 

 it ; that is to say, they show greater differentiation. 



If we were to begin at once to arrange all animals in a 

 regular graded series, on the basis of what we learn from 

 morphology and from embryology, we should have a difficult 

 task. The number of facts that it is necessary to know is so 

 considerable that even to-day systematic zoologists are not 

 agreed on important details of grouping animals in a com- 

 plete system of classification. Besides, the doctrine of evolu- 

 tion takes into account the fact that the phyla of animals 

 have not developed in a direct line, but as branches from a 

 previously existing stem. The whole system of animals 

 might be represented graphically by a series of stem and 

 branches resembling a tree ; but whereas in a tree we can 

 trace down one branch and out any other, in the diagram 

 suggested some of the branches would not be connected with 

 the stem at all, because the organisms which would stand in 

 the connecting places have disappeared both in living and 

 in fossil form. 



However, we can see very clearly that the Protozoa being 

 composed of a single cell are the lowest of all animals ; and 



