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THE POPULAR SCIENCE MONTHLY, 



From a sac-like body, moving freely through the water, and thus 

 exposed equally on both sides to the same environment, and therefore 

 bisymmetrical, we may suppose that all mollusks have been derived. 

 If such a free-moving body became fixed, unless as a stemmed Ascid- 

 ian, its parts would be diiFerently conditioned as to environment, 

 and the side more favored would outgrow the other. As the first part 

 of the snail's body to bend out of line with the axis is the intestinal 

 canal, we infer that this bend occurred far back in the snail's ancestry. 

 It occurs in the oyster. As the last organ to share the general twist 

 resulting from unequal growth of the sides is the heart, we infer that 

 displacement of this organ occmTed later down in the history of the 

 type. It does not occur in the oyster. 



Fig. 5. Asymmetry. Adult Snail: op, optic tentacle; oe, (ssophagus ; en, cephalic ganglion; 

 g, gizzard ; ,, htomach : I, liver ; L intestine (bent out of line with the axis of the body) ; h, 

 heart (auricle and ventricle not in line with axis or intestinal tube); v, vent. 



The first step toward a spiral-shelled gasteropod was taken in the 

 fii'St molhisk whose environment on one side was mud or rock, and on 

 the other water. Diftbrence of environment was the first factor in this 

 series of evolutions. Only this would induce one-sidedness, and act- 

 ing through long periods it might induce excessive one-sidedness. It 

 might carr}^ an oyster as far along in asymmetrical growth as the 

 partially rolled-up oyster called Gry])ha?a. When asymmetry came to 

 be of advantage to the animal. Natural Selection began and carried 

 it to greater excess, with the aid of other factors for Nature is too 



