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, 
Fic. 44—Prophysema primordiale, an extant gastrwa-form. (After 
Hiackel.) (A). 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 (7). Among the cells 
of the endoderm ( g) lie amoeboid egg-cells of large size (e). The ec- 
toderm (4) 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 
Calenterata. The preceding figures represent two 
