THE WONDERS OF LIFE 



various structures have been developed from one simple 

 fundamental form, as I have shown in my theory of the 

 gastrsea (1872). 



The older research into the origin of the nutritive 

 apparatus in the metazoa — especially its chief part, the 

 alimentary or gastric canal — had led to the erroneous 

 belief that in several groups of the metazoa it owed 

 its origin to very different growth-processes, and that 

 particularly in the higher vertebrates (the amniotes) it 

 was a comparatively late product of evolution. On the 

 other hand, the comparative study of the embryology of 

 the lower and higher animals led me thirty-four years 

 ago to the opposite conclusion, that a simple gastric sac 

 was the first and oldest organ of all the metazoa, and 

 that all the different forms of it had been developed from 

 this primitive type. I gave this view in my Biology of 

 the Sponges in 1872; and I developed and established it 

 in my Studies of the Gastrcea Theory in 1873. In the 

 latter book I also worked out the important conclusions 

 that follow from this monistic reform of the theory of 

 germinal layers for the phylogenetic natural classifica- 

 tion of the animal kingdom. I began with the con- 

 sideration of the simplest sponges (olynthtis) and cnidaria 

 {hydra). The whole body of these lowest and oldest of 

 the coelenteria is in essence nothing but a round, oval, or 

 cylindrical gastric vesicle, a digestive sac, the thin wall 

 of which consists of two simple layers of cells. The 

 outer layer (the ectoderm or skin-layer) is the covering 

 layer of the external skin (epidermis) ; it is the instru- 

 ment of sensation and movement. The inner layer of 

 cells (entoderm or gastric layer) serves for nutrition; it 

 clothes the simple cavity of the sac, which admits the 

 food by its opening and digests it. This opening is the 

 primitive mouth (prostoma or blastoporus), the inner 

 cavity itself the primitive gut (progaster or archenteron) . 

 I proved that there was the same composition in the 



222 



