SEGMENTAL BILATERAL SYMMETRY 225 



in the face of gravitation forces working for immobility 

 that the shape of the typical Fish is due. 



To a certain extent the Fish's locomotive powers are 

 based on those of ancestral medusoids ; but only to a 

 certain extent. The jelly-tissue in the bell of the medusoid 

 has been the forerunner of the Fish's muscle-tissue. But, 

 on the other hand, locomotion in a medusoid is almost, 

 we might say, a side-result ; bell-contractions being due 

 to the demand for a regular radial-canal circulation. The 

 medusoid's food surrounds it on all sides, and the organism 

 does not move along owing to direct-food-attraction, but 

 rather as the result of bell-contractions produced by the 

 intake of food and accumulation of impurities. In the 

 developing primitive Fish, however, the case would be 



\iiunut 



/•Jiff w s 



Fig. 68.— Showing how movement in the face of water-pressure 

 would produce a torpedo-like form in the evolving Fish. 

 A, a diagram representing the developing primitive Fish at 

 rest, evenly pressed on all sides by surrounding water. B, 

 roughly shows the lines of water-pressure A would experience 

 if moving through the water in the direction of the large arrow. 

 A being plastic, its elongating form would be made to take the 

 shape enclosed by the dotted lines in b ; that is, it would 

 become c. 



different, for its food would not be in a fine state of suspension 

 in the water of respiration, but was, we may presume, 

 coarsely particulate and scattered in its distribution. The 

 young Fish had therefore to develop the power of moving 

 in a definite line towards a localised source of food-attraction. 

 It would develop, in fact, a voluntary muscular system. 

 And it is to be noted that all voluntary movement would 

 be in the face of ever-acting gravity and water-pressure. 



The young evolving Fish would always have the same 

 part of its body leading the way as it moved through the 

 water, and during movement this part would encounter 

 strong resistance or friction. During rest the water would 

 press evenly on all parts of the body, but during movement 



15 



