202 MORPHOLOGICAL DEVELOPMENT. 



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 dif- 

 ferent individuals. We have but to remember that growing 

 clustered together, as Oysters do, they must interfere with 

 one another in various ways and degrees, to see how the 

 indeterminateness of form and the variety of form are 

 accounted for. 



Among the Gasteropods modifications of a more definite 

 kind occur. "In all Mollusks," says Professor Huxley, 

 " the axis of the body is at first straight, and its parts are 

 arranged symmetrically with regard to a longitudinal vertical 

 plane, just as in a vertebrate or an articulate embryo." In 

 some Gasteropods, as the Chiton, this bilateral symmetry 

 is retained the relations of the body to surrounding actions 

 not being such as to disturb it. But in those more numerous 

 types which have spiral shells, there is a marked deviation 

 from bilateral symmetry, as might be expected. " This 

 asymmetrical over-development never affects the head or 

 foot of the mollusk " : only those parts which, by inclosure 

 in a shell, are protected from environing actions, lose their 

 bilateralness ; while the external parts, subjected by the 

 movements of the creatures to bilateral conditions, remain 

 bilateral. Here, however, a difficulty meets us. Why is it 

 that the naked Gasteropods, such* as our common slugs, 

 deviate from bilateral symmetry, though their modes of 

 movement are those along with which complete bilateral 

 symmetry usually occurs? The reply is that their devia- 

 tions from bilateral symmetry are probably inherited, and 

 that they are maintained in such parts of their organization 

 as are not exposed to bilaterally-symmetrical conditions. 

 There is reason to believe that the naked Gasteropods are 

 descended from Gasteropods which had shells: the evidence 

 being that the naked Gasteropods have shells during the 

 early stages of their development, and that some of them 



