EVOLUTION OF THE VERTEBRATE 185 



opening to that at the end of a series of medusoids inevitably 

 had to be formed. This new opening became the mouth 

 of the organism. 



In segmental organisms the mouth develops at what 

 is termed the anterior end of the body ; that is, at the end 

 where the controlling nerve-centre develops, and which 

 habitually leads the way when the organism responds to 

 a source of attraction by moving towards it. The nerve- 

 centre itself, a mass of nerve-ganglia compressed together, 

 takes form as the result of terminal segmental compression 

 during development, this in turn being caused by resistance 

 encountered in movement under water-pressure. 



Now, in our hypothetical primitive serio-medusoid type 

 it is clear that bell-contractions would be the means 

 employed for active locomotion, and the convex end of the 

 series would be that which led the way during movement. 

 A developing embryo, therefore, which started life with 

 serio-medusoid potentialities, would inherit, so to speak, 

 this method of movement, and as a result its developing 

 serial segments would from the first encounter end-on re- 

 sistance at what we may call its convex extremity. Hence, 

 at this extremity terminal compression would be a feature 

 of development, and with it the massing of ganglia to form 

 a brain. Thus, while the mouth forms at the anterior end 

 where the brain takes shape, the brain forms at the end 

 of the organism, which for ancestral reasons has to lead 

 the way during movement. 



The development of a series of medusoids in such close 

 continuity as to become segments would produce an embry- 

 onic organism of a long, somewhat tubular shape, whose 

 walls, representing a continuity of medusoid bells, would 

 have the power of muscular contraction. Down the centre 

 of the body, suspended in the coelomic space, equivalent 

 to the successive spaces between ancestral medusoid bells 

 and manubrial structure, would run an alimentary tube. 

 This would represent the development in continuity of the 

 manubria of successive medusoids. 



The " inherited " opening of this tube would be at the 

 posterior or " concave " end of the organism (Fig. 58). 



Such an organism as that drawn below could never take 

 shape, given motile powers and ever-present water-pressure. 



