344 Evolution and Adaptation 



food. If this is true, then we find a symmetrical organism 

 becoming unsym metrical, and in consequence it takes advan- 

 tage of its asymmetry by using its right and left claws for 

 different purposes. 



More striking still is the difference in the size of the right 

 and left claws in a related form, Alpheus — a crayfish-like 

 form that lives in the sea. With the larger claw (Fig. 4 C) it 

 makes a clicking sound that can be heard for a long distance. 

 In some of the crabs the difference in the size of the two 

 claws is enormous, as in the male fiddler-crab, for example 

 (Fig. 4 B). One of the claws is so big and unwieldy that it 

 must put the animal at a distinct disadvantage. Its use is 

 unknown, although it has been suggested that it is a second- 

 ary sexual character. 



The asymmetry of the body of the snail is very conspicu- 

 ous, at least so far as certain organs are concerned. The 

 foot on which the animal crawls and the head have preserved 

 their bilaterality ; but the visceral mass of the animal, con- 

 tained in the spirally wound shell, lying on the middle of the 

 upper surface of the foot, is twisted into a spiral form. Many 

 of the organs of one side of the body are atrophied. The 

 gill, the kidney, the reproductive organ, and one of the 

 auricles of the heart have completely, or almost completely, 

 disappeared. The cause of this loss seems to be connected 

 with the spiral twist of the visceral mass. One of the conse- 

 quences of the twisting has been to bring the organs of the 

 left side of the body around the posterior end until they come 

 to lie on the right side, the organs of the original right side 

 being carried forward and there atrophying. 



There is another remarkable fact connected with the asym- 

 metry of the snail. In some species, Helix pomatia, for 

 example, the twist has been toward the right, i.e. in the 

 direction which the hands of a watch follow when the face 

 is turned upward toward the observer. Individuals twisted 

 in this direction are called dextral. Occasionally there is 



