1898] - / .V.V ULA TE . \ Nt ES Til Y OF THE \ 'Ell TEH 11. 1 TA 2 3 



monstrous, the end is gained by a waist (or, as in Scorpio, a perforated 

 diaphragm, which is only a waist with the infolded external faces 

 fused together). From the arrangement of the muscles, it appears 

 that the narrow neck of this waist can he constricted when neces- 

 sary. Xo such method of protection, of course, is conceivahlc in the 

 case of the primitive vertebrates, in which a great part of the dorsal 

 region of the body had to be protected from the ventral half of the 

 same region. That protection was as necessary as it is in the case 

 of the arachnids, we may surely believe if the rotundity of the body 

 of the tadpole is any sort of repetition of the state of periodic dis- 

 tension of our early ancestors, although of course in the tadpoles 

 later modifications, such as the coiling of the intestine and the for- 

 ward movement of the vent, are already superposed. 



The method of protection which actually was adopted — reading 

 now from the embryological record — seems to have been as follows. 

 A dorsal strip of the alimentary canal thickened and eventually 

 separated off as the functions of the canal demanded free play to 

 cope with the increasingly difficult digestive problems which the 

 developing mouth and teeth, in their quest of new things to devour, 

 continually sent down for solution. This thickening of a dorsal 

 strip of the alimentary canal may perhaps again be referred to the 

 downward pressure of the food at all times, whether the alimentary 

 canal was distended or not. The dorsal epithelium would only be 

 seriously pressed upon in the condition of actual distension, the 

 ventral would be subjected to pressure whenever there was any solid 

 food to rest upon it. 



It is conceivable that this dorsal strip of endoderm might have 

 remained a protective plate if its sole function had been to screen 

 the neuro-muscular system from the distension and churning activity 

 of the alimentary canal. But, at the same time, it served another 

 and almost equally important purpose which led to its stiffening 

 longitudinally into an elastic rod. It is, indeed, horrible to con- 

 template the possible fate of our unfortunate ancestors if, while the 

 ventral half were fully distended with food, some sudden stimulus 

 compelled the dorsal half violently to contract, as we know the 

 modern leech can contract, to less than a quarter of its length. A 

 stiff rod along the back alone could avert such a catastrophe. 

 Hence I would suggest that the protective strip of endoderm 

 stiffened longitudinally as it was progressively differentiated from the 

 alimentary canal, and further, narrowed as it thickened, so as to 

 permit of free serpentine movements of the body. We may then 

 leave it as an elastic rod protecting on the one hand the spinal cord 

 — which, as we shall see, must have been concurrently developed — 

 from functional disturbance by the alimentary canal, and on the 

 other, the alimentary canal from possible mechanical injury due to 



