THE WONDERS OF LIFE 



labiate and papilionaceous flowers, etc., in the plant world: 

 and in the five-rayed "irregular" echinoderms, the bilateral 

 sea-urchins (spatangida, clypeastrida, A-f. 30) in the animal 

 world. In these cases the bilateral symmetry is recognizable at 

 the first glance, as is also the radial structure, or the composition 

 from three to five or more raylike parts (parainera), which are 

 arranged bilaterally round a common central plane. 



The bilateral-symmetrical type is general among the higher 

 animals which move about freely. The body consists of two 

 antithetic parts {antitnera) , and has no trace of radial structure. 

 In the free moving, creeping, or swimming animals (vertebrates, 

 articulates, mollusks, annelids, etc.) the ventral side is under- 

 neath, against the ground, and the dorsal side upward. This 

 form is clearly the most useful and practical of all conceivable 

 types for the movement of the body in a definite direction and 

 position. The burden is equally distributed between the two 

 sides (right and left) ; the head (with the sense organs, the 

 brain, and the mouth) faces frontward and the tail behind. 

 For thousands of years all artificial vehicles (carts on land and 

 ships in water) have been built on this type. Selection has 

 recognized it to be the best and preserved it, while it has dis- 

 carded the rest. There are, however, other causes that have 

 produced the predominance of this type in green leaves — the 

 relation to the supporting stalk, to the sun-light that falls from 

 above, etc. 



Special notice must be taken of those bilateral forms 

 which were originally symmetrical (by heredity), but 

 have subsequently become asymmetrical (or of unequal 

 halves), by adaptation to special conditions of life. The 

 most familiar example among the vertebrates are the 

 flat-fishes (plciironectides), soles, flounders, turbots, etc. 

 These high and narrow and flattened boney-fishes have 

 a perfect bilateral symmetry when young, like ordinary 

 fishes. Afterwards they form the habit of laying on 

 one side (right or left) at the bottom of the sea; and in 

 consequence the upper side, exposed to the light, is dark 

 colored, and often marked with a design (sometimes 

 very like the stony floor of the ocean — a protective 

 coloring), while the side the flat-fish lies on remains 

 without color. But, what is more curious, the eye 



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