EVOLUTION 1071 



are considered, this superficial radial symmetry does not usually 

 hold quite good for the internal structure of the body. Thus there 

 are only two cuts that will divide a sea-anemone into two precisely 

 similar halves, namely, {a) the vertical cut through the plane of the 

 two ciliated gullet-grooves (siphonoglyphs), and (b) the vertical cut 

 through a plane at right angles to the first. Similarly, though a sea- 

 urchin is radially symmetrical — ^not only superficially, but in the 

 5-rayed disposition of the nervous, water-vascular, blood-vascular, 

 and reproductive systems — there is only one plane which will 

 divide it into precisely mirroring halves, namely, the plane that 

 bisects the madreporic plate and anus vertically. No other cut would 

 give half of the madreporic plate to each of the halves of the sea- 

 urchin. 



To come to the point, however, radial symmetry is adapted {a) to 

 sedentary life, as when the sea-anemone waits for food to drop into 

 the circle of its outspread tentacles, and (b) to easy-going (Plankton) 

 life in the open waters, where it matters little in what direction the 

 animal moves, as is well illustrated by jellyfish. 



Bilateral symmetry, which occurs in the majority of animals from 

 worm to man, shows a right and left side; and the body can be 

 halved in a longitudinal median dorso-ventral plane and in no other. 

 This kind of symmetry, where one end of the body takes the lead in 

 locomotion, is adapted for more energetic life than radial symmetry 

 allows. It is suited for pursuing booty and mates, and for avoiding 

 enemies. It is to be correlated with the evolution of head brains, 

 that is to say variations in the direction of having neurons at the 

 head end especially. In the testing of variations that always goes on 

 in everyday life, those variants that had most concentration of 

 neurons at the anterior end would have an indubitable advantage. 

 This again should be correlated with the "metabolic gradient" (q.v.), 

 which is so clearly seen in the simplest Planarian worms where 

 the area of intensest metabolism is at the head end. It is not too 

 much to say that if Planarian worms, or some similar pioneers, had 

 not begun to move with their head end foremost, Man would never 

 have been able to tell his right hand from his left. 



It is interesting to notice that some animals which are sedentary 

 or sluggish in adult life, but active as larvae, become radial as adults, 

 although they were bilateral in the larval phase. Thus the free- 

 swimming Open-Sea larvae of the shore sea-urchins and brittle-stars 

 are bilateral. Bilaterality is adaptive to vigorous locomotion. 

 Whether there is any adaptiveness in acquiring asymmetry, such 

 as snails illustrate, we do not know; but it is worth noticing the case 

 of bony flat-fishes, like plaice and sole, which are symmetrical as 

 larvae, but markedly asymmetrical as adults, having both eyes, for 

 instance, on the up-turned pigmented surface. The young larvae 

 are shaped like those of "round" fishes such as haddock and herring, 



