184 MORPHOLOGICAL DEVELOPMENT. 



? 251. Among tlie Mollusca we find more varied relationa 

 between shapes and circumstances. Some of tliem are 

 highly instructive. 



MoUusks of one order, the Pterojjoda, 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- s}Tn- 

 metncal shape harmonizes with itsbilateralty- 

 s}Tnmetrical conditions is sufficiently obvious. 

 j^7^ ^ Among the Lamellibranchiata, we have 



diverse forms accompanying diverse modes ot 

 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 s^onmetrical 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 ihe 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 difierenti- 

 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 anticij)ated under the circumstances. "Where 

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

 attachment, while the other valve is alwa3's upwards, or next 

 to the envii'oning 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 



