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, 



FlG. 44. Prophysema primordiale, an extant gastroM-form. (After 

 Hackel.) (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 month (/). Among the cells 

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

 toderm (A) 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 

 Ccelenterata. The preceding figures represent two 



