140 Darwin, and after Darwin. 



kind with a hole in it, and push in one side with our 

 fingers till internal contact is established all round, by 

 then holding the indented side downwards we should 

 get a very fair anatomical model of a gastraea form, 



A li 



Fig. 44. — Propkysema primordiale, an extant gastrwa-form. (After 

 Hackel.) (Aj. External view of the whole animal, attached by its foot 

 to seaweed. (B). Longitudinal section of the same. The digestive 

 cavity {d) opens at its upper end in the mouth (w). Among the cells 

 of the endoderm {g) lie amoeboid egg-cells of large size (e). The ec- 

 toderm (Ji) is encrusted with grains of sand, above the sponge spicules. 



such as is presented by the adult condition of many 

 of the most primitive Metazoa — especially the lower 

 CcelenUrata. The preceding figures represent two 



