184 MORPHOLOGICAL DEVELOPMENT. 



§ 251. Among the Mollusca we find more varied relations 

 between shapes and circumstances. Some of them 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 bilaterally- 

 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 differenti- 

 ating 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 

 valves become unlike. This we do find : witness the Oyster. 

 In the Oyster, too, we see a further irregularity. There is a 

 great indefiniteness of outline, both in the shell and in the 

 animal — an indefiniteness made manifest by comparing 

 different individuals. We have but to remember that growing 

 clustered together, as Oysters do, they must interfere with 



