EVOLUTION OF THE VERTEBRATE 189 



however, would not suffice for the needs of the developing 

 organism, for not only would oxygen have to be absorbed 

 in large quantity, but it would have to be carried to the 

 tissues of the segments anterior and posterior to the gill- 

 clefts. Moreover, impurities in the closed circulation would 

 have to be borne to the branchial vessels for excretion. 

 Thus a circulation of the contents of the closed vessel-system 

 would be called for. No doubt the developing primitive 

 Fish would help to maintain a circulation in its vessels by 

 serial body-wall contractions, but this means would be 



Fig. 60. — Formation of mouth, and gill-clefts, of primitive 

 Fish. B, i, light and left sides of head which represents many 

 compressed segments, whose circular and longitudinal canals 

 have become bv, bloodvessels of a closed circulation. The 

 powerful contractions of posterior E and i are supposed to have 

 caused dilatation of the digestive tube and finally breaches 

 at the site of the arrows. The branchial loops, I, represent 

 ancestral circular canals, across which, instead of through 

 which (as in medusoids) the respiratory water passes. Posterior 

 to them is the representative of an ancestral circular canal 

 adapted for tissue nourishment. The figure is ultra-diagram- 

 matic. 



insufficient and spasmodic. The organism would develop 

 a heart. 



We seem to get a hint of the acting causes behind heart 

 formation from the free-swimming medusoid, for it would 

 appear that the regularly recurring accumulation of waste- 

 products, carbonic acid, etc., in the medusoid canal system 

 is the exciting cause of the bell-contractions, and it is 

 reasonable to suppose that the same factor in the closed 

 vessel-system of the developing primitive Fish would produce 

 parallel results. These can be recognised in the regular 

 gill-contractions, but as we have seen that gill action alone 



