192 THE EVOLUTION OF CONTINUITY 



and necessitating circulation, were a primitive blood of 

 sorts just as the canals were primitive bloodvessels com- 

 municating with the stomach cavity. The development in 

 continuity of successive medusoid canal systems, and the 

 cutting these off from communication with the digestive 

 canal resulted in the closed circulation of the segmental 

 organism. But this is, we might say, inevitable evolution. 

 But Gaskell's theory pictures a nervous system (which 

 had evolved harmoniously with the digestive and other 

 systems of the body it governed) as suddenly overgrowing 

 and practically obliterating the all-important alimentary 

 tract on which its own normal development depended. 

 This might occur as uncontrolled growth, or as a nervous 

 neoplasm, but not as a modification, inevitable, and necessary 

 for evolutionary advance. We cannot beUeve that Nature 



Fig. 61. — The evolution of the segmental nerve-chain, g, the 

 ganglia of successive medusoid rings linked together. 



could make a new alimentary tract where no inherited 

 alimentary tissues or potentialities were present. 



A simple explanation of the dorsal position of the 

 vertebrate cord offers itself through the theory that 

 Vertebrates and organisms of Annelid type, while both of 

 medusoid ancestry, evolved on separate lines from the 

 beginning. In a word, it is to the effect that the method 

 followed in the " linking up " in continuity of the developing 

 representatives of successive medusoidal ganglia has differed 

 in the two cases. 



The nerve-ganglia of a medusoid are situated in the 

 bell-rim, and the linking up of the successive ganglia in a 

 continuous medusoid series would make a number of 

 longitudinal ganglionic chains, as in Fig. 61. 



This we take to be the basic plan on which depended 

 alike the evolution of the Annelid and the Vertebrate. In 

 a medusoid the ganglionic ring in the bell-rim surrounds 



