THE PHYLOGENY OF THE SOUL. 163 



which has a simple cavity with one opening — the 

 primitive gut and the primitive mouth. The wall of 

 the digestive cavity is formed of two simple layers of 

 cells, or epithelium, the inner of which — the gut-layer 

 — is responsible for the vegetal activity of nourish- 

 ment, while the outer, or skin-layer, discharges the 

 animal functions of movement and sensation. The 

 homogeneous sensitive cells of the skin-layer bear 

 long, slender hairs or lashes (cilia) , by the vibration 

 of which the swimming motion is effected. The few 

 surviving forms of gastraeads, the gastraeniaria (tricho- 

 placidce) and cyemaria (orthonecticUe) , are extremely 

 interesting, from the fact that they remain through- 

 out life at a stage of structure which is passed by all 

 the other metazoa (from the sponge to man) at the 

 commencement of their embryonic development. As 

 I have shown in my Theory of the Gastraa (1872), a 

 very characteristic embryonic form, the gastrula, is 

 immediately developed from the blastula in all the 

 tissue-animals. The germinal membrane (blas- 

 toderm), which represents the wall of the hollow 

 vesicle, forms a depression at one side, and this soon 

 sinks in so deep that the inner cavity of the vesicle 

 disappears. The half of the membrane which bends 

 in is thus laid on, and inside, the other half ; the 

 latter forms the skin-layer, or outer germinal layer 

 (ectoderm or epiblast), and the former becomes the 

 gut-layer, or inner germinal layer (endoderm or hypo- 

 blast). The new cavity of the cup-shaped body is the 

 digestive stomach-cavity [the prog aster), and its open- 

 ing is the primitive mouth (or prostoma)} The skin- 

 layer, or ectoderm, is the primitive psychic organ in 



1 Cf. Anthropogeny and Natural History of Creation. 



