188 THE EVOLUTION OF CONTINUITY 



contraction following relaxation. In the second place, the 

 formation of an anterior mouth would bring about the 

 closure of the developing circulatory system. 



This system in a continuous medusoid series would be 

 the linked-up canal-systems of the successive medusoids, 

 and would only communicate with the manubrial tube in 

 the convexity of the anterior terminal medusoid-segment, 

 and it seems clear that the formation of a mouth at this 

 spot would cut off the terminal communicating radial canals 

 from the digestive tube, and leave the embryo with a 

 developing closed vascular system. This can be made clearer 

 in the next figure (Fig. 59). 



It may well be that terminal compression of the leading 

 segments would assist in closing the circulatory system ; 



Fig. 59. — A, diagram of " circulatory system " of imaginary 

 serio-modusoid organism, showing how the manubrial digestive 

 tube would only communicate with the linked-up canal-systems 

 at the anterior extremity. B, showing how the formation of 

 an anterior mouth could cut off the radial canals, m, m, the 

 digestive tube ; r, r, the radial canals at anterior end of the 

 organism. 



and it also occurs to one that this factor might assist in the 

 formation of gill-clefts, the compressed developing anterior 

 segments exercising specially strong contractions on the 

 digestive tube and coelom in their neighbourhood. 



This apart, however, the point to which we return is 

 that the formation of an anterior mouth would close the 

 developing vascular system, and that the indrawn water of 

 respiration would of necessity pass over and not through 

 the primitive branchial vascular rings (Fig. 60). 



But simple branchial loops, such as in the diagram, 

 would not offer a sufficiently large surface for the absorption 

 of the oxygen passing over them, and it is quite probable 

 that the gas itself would directly stimulate vascular branching 

 and the formation of branchial capillaries. This alone, 



