EVOLUTION OF THE VERTEBRATE 187 



come into play powerful physical forces. There would be 

 a moulding of the developing and plastic, not of the mature 

 and rigid. There would not be a narrowing and tapering 

 of an already formed Fish-like structure, nor of an already 

 formed manubrial orifice, but a moulding of form and orifice 

 during development. As serio-medusoid potentialities tried 

 to realise themselves the new plan would be made to evolve, 

 and modification, once started, would itself be active in 

 causing further modification. 



For example, the commencing tapering of body form 

 and the narrowing of the inherited orifice of the digestive 

 tube would result in the accumulation of impurities, which 

 in turn would induce strong contractions of the body-wall 

 segments. And these contractions would raise the pressure 

 in the coelom and in its contained digestive tube. We 

 can thus not unreasonably picture the rise of pressure in 

 the digestive tube (increased by the insufficient size of the 

 outlet at the posterior end), resulting in a breach in the 

 tube walls and the body- walls at the junction of certain 

 developing segments. That is, the very need for oxygen 

 would bring about gill-formation. In addition, the same 

 internal pressure might result in the formation of a new 

 opening at the anterior end of the organism, whose alimen- 

 tary tract would now have a mouth as well as an anus ; 

 though it is possible that the mouth was formed not by 

 positive pressure from within during body-wall contraction, 

 but by external water-resistance on the anterior end during 

 movement, assisted by internal negative pressure during 

 body-wall relaxation : was pushed and sucked in, and not 

 pushed out. At the same time, the compression of the 

 anterior developing segments would be going on, a nerve- 

 centre taking shape, and all the modifications occurring 

 which would exert a mutual balancing influence while 

 evolving to become new characters. 



The formation of the mouth and gill-slits or branchial 

 clefts of the developing embryo would have important 

 results. For in the first place the muscular relaxation of 

 the walls of the anterior segments would suck in water at 

 the new mouth-opening, and this water would pass but a 

 short distance down the alimentary tract to escape at the 

 gill-slits. The escape would be assisted by the muscular 



