The Classification of Animals. 185 



of food captured by and passed on to them by the more super- 

 ficial cells. To this it may, of course, be answered that, after 

 all, each cell does, as a matter of fact, act independently in 

 taking up food particles; that there is no throwing out of a 

 digestive fluid by the cells at large and a subsequent absorption 

 of the digested food, as in the stomach and intestines of the 

 higher animals. But in Hydra the individual cells lining the 

 enteron do take up separately particles of food ; and this process 

 of assimilation appears to go on even in the highest animals 

 — witness the absorption of fat drops by the intestinal cells. 



Still, on the whole, the second definition of the Metazoa is 

 correct. 



To these two definitions is sometimes added a third — that 

 sexual generation is universal. This, however, is only a 

 difference of degree, and the intermediate stages offered by 

 such forms as the colonial Protozoa appear to do away with 

 any marked distinction of this kind. (See the remarks on 

 p. 183.) 



We can thus primarily divide the animal world into two 

 great divisions — the Protozoa and the Metazoa. 



The vast assemblage of animals which fall into the second 

 division show immense differences among themselves, which 

 permit of a further subdivision. If we compare the lowest 

 representative of the Metazoa with which we are concerned 

 here, the hydra, with any of the higher forms, we find it to 

 be marked off by two features of great importance, which are 

 related to each other. The body of the hydra is built up of 

 two layers of cells only, which surround a central cavity. In 

 all the remaining Metazoa not only are there these two layers 

 present, but also an interpolated layer, which is more or less 

 excavated into a cavity, or set of cavities, lying between the 

 ectoderm and the endoderm; to this cavity or cavities the 

 term ccRlom is applied, and the animals which possess it are 

 called Ccelomata. 



The group of Metazoa typified by hydra, and the group 

 typified by any of the other forms, used to be distinguished as 

 Diploblastica, or two-layered animals, and Triploblastica, or 



