EVOLUTION OF THE ATYPICAL FISH 199 



contractions would be stopped sharply at the junction of 

 body-sac and head ; and the fluid contents between the 

 digestive tube and body-walls, being unable under pressure 

 to escape because of the obstructing head, would force an 

 opening by which to issue. The most likely spot to give 

 way would be where the body joined the head, for here the 

 pressure would be greatest. As a result of the breach, 

 seawater would then pass in and out to circulate round the 

 digestive tube suspended in the coelomic cavity (Fig. 65). 

 That is, we suppose the same forces to have been at work 

 in the formation of the developing primitive Cephalopod's 

 gill-clefts as in the case of the evolving Pish ; the difference 



Fig. 66. — The evolution of the Cephalopod's gill-cleft. Show- 

 ing how, as development proceeded, the body-wall contrac- 

 tions, by acting on the fluid contents of the body-sac, would 

 cause a rupture in the region indicated as x. a represents the 

 state of relaxation ; b, the effects of contraction. 



being that the clefts took shape at what we might call the 

 opposite ends of the ancestral medusoid-series. 



As in the case of the evolving primitive Fish, the 

 vascular system of the Cephalopod would be ancestral 

 water-vascular canal systems which bad been cut off in 

 their development from all communication with the alimen- 

 tary tube ; and in the region of the gill-cleft the indrawn 

 oxygenated water would stimulate representatives of 

 ancestral circular canals to act as branchial loops, and to 

 form the fine capillaries of primitive gills. 



In the primitive Fish a mouth had to take form which 

 communicated with the gill-clefts ; the inherited alimentary 



