EVOLUTION OF THE ATYPICAL FISH 197 



would have resulted when development under compression 

 took place ? With the help of the modern cuttlefish we 

 can, to a certain extent. 



Let us start, then, with our primitive serially-medusoid 

 type, noting that it was not motile, but most probably 

 attached to the sea-bed at its convex extremity. This 

 requires no stretch of imagination, for in the developing 

 Aurelia (Fig. 54) we have practically such an organism. 

 If a fertilised ovum, with developmental potentialities of 

 a strobilar kind, such as Aurelia possesses, were obliged to 

 develop under strong pressure conditions, the Cephalopod's 



JL 



Fig. 63. — Terminal compression in the evolving atypical Fish. 

 A, the inherited developmental potentialities of the fertilised 

 ovum which was caused to develop under special pressure 

 conditions. The figure is virtually a " strobila," anchored to 

 the sea-bed at 6. m, the digestive tube of manubrial deriva- 

 tion, communicating with the water-vascular system, c, at d. 

 Terminal compression during development, as indicated by the 

 arrows, could, other results apart, give the result shown in b. 



plan would, it is suggested, be the result. In the absence 

 of movement with convex developing end leading there 

 would be no terminal compression of this region as in 

 typical Fish evolution ; but the concave end would be the 

 one to suffer compression. For as the strobilar plan tried 

 to realise itself through growth towards the surface of the 

 water, the potentially concave end would be subjected to 

 overlying and surrounding water-pressure, and it would 

 be this region which would be terminally compressed in 

 its development. In other words, a head would take shape 

 at what would represent the tail end in the evolving typical 

 Fish (Fig. 63). 



