186 THE EVOLUTION OF CONTINUITY 



In the first place, forward movement in water-pressure 

 conditions would inevitably mould the body on stream-lines 

 (see Fig. 68, in chapter on Symmetry), and cause a tapering 

 of form towards the posterior end. But, more important 

 than this, the systems of digestion, respiration, and circula- 

 tion, suitable for a single medusoid, would never suffice 

 for a long series of medusoidally derived segments. The 

 medusoid's mouth is also its anus, and the stomach and 

 canals can be filled and emptied through the one opening, 

 but it is clear that one long manubrium with one terminal 

 orifice could never admit the nourishment necessary for a 

 long series of segments, nor allow the escape of impurities ; 

 especially so when it is remembered that the whole series 



Fig. 58. — Diagram in connection with mouth formation in the 

 primitive Fish. Showing how the " inherited " mouth would 

 be at the posterior end of the developing organism, special 

 factors apart. A, anterior end with terminally compressed 

 developing segments and mouth ; b, brain mass ; w, con- 

 tractile walls, the segments, s, of which represent ancestral 

 medusoid bells ; c, ccelom ; m, digestive tract, whose one 

 inherited opening is at the concave posterior end, p. This 

 diagram is not offered as a developmental stage. 



of linked-up canal systems would only communicate with 

 the digestive tube by the radial canals of the terminal 

 leading segment (Fig. 59, A). Further, the development of 

 tapering form would reduce the manubrial orifice to a 

 minimum. 



Our conclusion is that the inherited manubrial orifice 

 became, or developed as, the anus of the primitive Fish. 



It is to be remembered that the primitive Fish must 

 have evolved suddenly ; that a fertilised ovum with 

 " strobilar " potentialities must have been so acted on during 

 its development that new characters appeared all at once. 

 Thus, during its development, the elongating embryonic 

 mass would be compressed in close segmental Continuity, 

 and as it moved with its convex end leading there would 



