66 THE EVOLUTION OF MAN. 



lines of all these classes of animals have developed phylo- 

 genetically from the same parent-form. This most signifi- 

 cant primaeval parent-form is the Gastraea. 



The Gastrea was at any rate already present in the 

 sea during the Laurentian period, and by means of its 

 vibratory fringe hurried about in the water, just like the 

 yet extant free-moving ciliated gastrulae of this age. Pro- 

 bably the primaeval Gastrsea, which has been extinct for 

 many millions of years, differed from the living gastrula 

 of the present day only in some unessential point. On 

 grounds derived from Comparative Anatomy and Ontogeny, 

 the explanation of which would lead us too far, we may 

 assume that the Gastraea had already acquired sexual re- 

 production, and did not only propagate its species asexually 

 (by division bud-formation or spore-formation), as was 

 probably the case with the four preceding ancestral stages. 

 Presumably, single cells of the primary germ-layers as- 

 sumed the character of egg-cells, others that of fertilizing 

 seed-cells. (Of. Chapter XXV.) This hypothesis is founded 

 on the fact that sexual reproduction is yet met with in the 

 same simple forms in the lowest Plant- Animals (Zoophyta), 

 especially in the Sponges. 



Two small animal forms are especially interesting in 

 their bearing on this aspect of the Gastraea theory. They 

 have as yet been little observed, but of all extant animals 

 they are most nearly allied to the primaeval Gastraea, and 

 may therefore be called "the Gastraeads of the present 

 day." 142 One of these animals, Haliphysema (Figs. 180 and 

 181), has been described by Bowerbank as a Sponge ; the 

 other, Gastrophysema, by Carter as a Rhizopod (as " Squa- 

 mulina "). The entire mature body of the developed person 



