16 



GENERAL ZOOLOGY 



posed of one single cell; another, Metazoa, by grouping 

 together those whose bodies are multicellular. 



2. Diplo- or Triplo-blastic (Fig. 13). Animals made up 

 of many cells show differences in arrangement. In the 

 sponges the cells do not form layers but in other meta- 

 zoans they form definite sheets. We may have, therefore, 



FIG. 13. Sections across the bodies of Hydra and Planaria showing diplo- 

 blastic and triploblastic structure. In the first there are two layers of cells be- 

 tween the digestive cavity and the outside; in the second, there are three. 



diploblastic animals with two layers of cells the ectoderm 

 and entoderm; or triploblastic animals with three ectoderm, 

 mesoderm, and entoderm. These primitive sheets of cells 

 which appear in the development of most Metazoa are 

 called the germ layers. 



3. Body Metameric or Non-metameric (Fig. 14). The 

 bodies of many animals show a condition of metamerism, 



FIG. 14. A slug and a leech, showing the absence and presence of metamer- 

 ism. The slug has no subdivisions in the body, but in the leech the body is made 

 up of a series of similar parts. 



or segmentation. Similar parts are arranged one after 

 the other and form a sort of a chain. Each of the seg- 

 ments has a more or less complete set of organs, and these 

 are repeated in the successive segments of the body. Man 



