GRADE II. _The Two-layered, or Diploblastic Animals._--The type of these usually chosen is _Hydra_, a two-layered animal, which is further described on p. 54. A section through Hydra (Fig. 4) shows (1) 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 scientific ideas. The two-layered body of Hydra encloses a hollow digestive space; from this the Coelenterata receive their name, which means "possessing a hollow space only, by way of intestines." The name of Acoelomata, animals without a body-cavity, has therefore been given to the Coelenterata 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 Mesoderm. There are the beginnings of this in the Coelenterata and Sponges, but here it is further established. It includes a very thick layer of muscles. [Illustration: FIG. 5.--Diagrammatic plan of section cut through an Earthworm to show the position of the three body-layers and the body-cavity (Grade IV.). _Sk_, skin; _al_, glandular lining of the alimentary canal; _w_, muscular wall of body; _w'_, muscular of intestine, both belonging to the third layer or mesoblast; _b.c._, body-cavity (shaded); _al.c._, cavity of alimentary canal (shaded); _n_, nerve.]