THE GENERAL SHAPES OP ANIMALS. 201 



form, by the action of unsymmetrically-bilateral conditions. 

 A further illustration is supplied by Bopyrus, Fig. 271 : a 

 parasite which lives in the branchial chamber of prawns, and 

 whose habits similarly account for its distorted shape. 



251. Among the Mollusca we find more varied relations 

 between shapes and circumstances. Some of these relations 

 are highly instructive. 



Mollusks of one order, the Pteropoda, swim in the sea 

 much in the same way that butterflies fly in 

 the air, and have shapes not altogether unlike 

 those of butterflies. Fig. 272 represents one 

 of these creatures. That its bilaterally-sym- 

 metrical shape harmonizes with its bilater- 

 ally-symmetrical conditions is sufficiently 

 obvious. 



Among the Lamellibranchiata, we have 

 diverse forms accompanying diverse modes of life. Such 

 of them as frequently move about, like the fresh-water 

 Mussel, have their two valves and the contained parts 

 alike on the opposite sides of a vertical plane: they are 

 bilaterally symmetrical in conformity with their mode of 

 movement. The marine Mussel, too, though habitually 

 fixed, and though not usually so fixed that its two valves are 

 similarly conditioned, still retains that bilateral symmetry 

 which is characteristic of the order; and it does this because 

 in the species considered as a whole, the two valves are not 

 dissimilarly conditioned. If the positions of the various 

 individuals are averaged, it will be seen that the differen- 

 tiating actions neutralize one another. In certain 

 other fixed Lamellibranchs, however, there is a considerable 

 deviation from bilateral symmetry; and it is a deviation of 

 the kind to be anticipated under the circumstances. Where 

 one valve is always downwards, or next to the surface of 

 attachment, while the other valve is always upwards, or next 

 to the environing water, we may expect to find the two 



