THE ANCESTRY OF VEHTEBliA'lE.S 



297 



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improbable that 

 such a change in 

 position could have 

 occurred; but 

 when we recall that 

 the ancestors of 

 vertel^rates, like 

 modern prechor- 

 dates, probably 

 assumed a vertical 

 position, it is not so 

 cUfRcult to under- 

 stand how the tran- 

 sition may have 

 occurred. The 

 mouth must have 

 changed from the 

 neural side (on 

 which the nerve 

 cord lies) to the 

 hffimal side (that of 

 the heart), and we 

 find that in the 

 larva of Amphioxus 

 it lies on neither 

 the neural nor 

 haemal side, but 

 halfway between ; a 

 significant position 

 if it indicates a 

 stage in the ances- 



tral movement of the mouth from the neural to the haemal side. 



