CLASSIFICATION 37 



Animals. The type of these usually chosen is 

 Hydra, a two-layered animal, which is further de- 

 scribed on p. 54. A section through Hydra (Fig. 

 4) shows (i) the outer or skin layer of cells, called 

 the ectoderm, and (2) the inner or stomach layer 

 of cells, called the endoderm (literally outer skin 

 and inner skin). The clear recognition of the 

 primary body-layers of the simpler invertebrates 

 as identical with the primary body-layers of the 

 embryo of higher forms, is largely owing to the 

 teaching of Professor Huxley, the importance of 

 whose work on this and in many other respects, 

 is little guessed at by many readers who know 

 his name merely as a popular exponent of scien- 

 tific ideas. The two-layered body of Hydra en- 

 closes a hollow digestive space ; from this the 

 Coelenterata receive their name, which means 

 " possessing a hollow space only, by way of in- 

 testines." The name of Accelomata, animals 

 without a body-cavity, has therefore been given 

 to the Ccelenterata and sponges. The meaning of 

 the term body-cavity will be explained in the next 

 paragraph but one. The Hydra, like all animals 

 of its grade, and all those of the succeeding grades, 

 reproduces itself by means of ova or egg-cells, 

 and spermatozoa which fertilize them. 



GRADE III. The Triploblastic Animals without 

 Body-Cavity. This is a small section including 

 only some of the lowest worms, such as the forms 

 called Planarians. Between the Ectoderm and 

 Endoderm lies an intermediate layer the Meso- 

 derm. There are the beginnings of this in the 

 Ccelenterata and Sponges, but here it is further 

 established. It includes a very thick layer of 

 muscles. 



GRADE IV. The Ccelomata or Triploblastic Ani- 



