38 THE STORY OF ANIMAL LIFE 



mats with a Body-Cavity. This grade includes all 

 the remainder of the animal kingdom. As an ex- 

 ample of it, we may take the Common Frog. If 

 we open from the lower surface the dead body of 

 a frog, we first cut through the skin, next the 

 muscles; then we come to the viscera, lying 

 neatly packed in a cavity from which we can dis- 

 iodge them. This cavity is the Body-Cavity. 



The skin corre- 

 spends with the 

 ectoderm of Hy- 

 dra5 although it is 

 a vastly more com- 

 plicated affair. 

 The glandular lin- 

 ing of the alimen- 

 tary canal corre- 

 sponds with the 

 endoderm of Hy- 

 section dra ; although this, 

 too, is a more 

 complicated affair. 

 The mass of the 

 body, lying be- 

 tween these two 



FlG. 5. Diagrammatic plan 

 cut through an Earthworm to show 

 the position of the three body-layers 

 and the body-cavity (Grade IV. ). Sk, 

 skin ; a/, glandular lining of the ali- 

 mentary canal ; zv, muscular wall of 

 body ; w ', muscular of intestine, both 

 belonging to the third layer or meso- 



blast ; b.c., body - cavity (shaded) ; 

 a/.c. } cavity of alimentary canal (sha- 

 ded) ; , nerve. 



layers, is consid- 

 ered to correspond 

 somewhat with the 

 mesoderm of Grade III., and has received the 

 collective term of Mesoblast. This description 

 applies equally to the earthworm, for the higher 

 worms differ immensely from the lower worms, 

 and stand on a level with more impbrtant mem- 

 bers of the animal kingdom (see Fig. 41, p. 139). 

 The body-cavity may be formed in different 

 ways in different animal groups; but there is 



