THE GENERAL SHAPES OF ANIMALS. 185 



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

 mdeterminateness of form and the variety of form are 

 accounted for. 



Among the Gasteropoda, 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 verti- 

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

 In some Gasteropods, as the Chiton, this bilateral spn- 

 metry is retained — the relations of the body to surround- 

 ing actions not being such as to disturb it. But in those 

 more numerous types that have spiral shells, there is a 

 marked deviation from bilateral symmetry, as might be ex- 

 pected. " This asymmetrical over- development never affects 

 the head or foot of the moUusk : " 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 difiiculty meets us. AYhy is it 

 that the naked Gasteropods, such as our common slugs, 

 deviate from bilateral sjonmetry, though their modes of 

 movement are those along with which complete bilateral 

 symmetry usually occurs ? The reply is, that their devia- 

 tions from bilateral sj^mmetry are probably inherited, and 

 that they are maintained in such parts of their organiza- 

 tion as are not exposed to bilaterally-symmetrical conditions. 

 There is reason to believe that the naked Gasteropods are 

 descended from Gasteropods that had shells : the evidence 

 being that the naked Gasteropods have shells during the 

 early stages of their development, and that some of them 

 retain rudimentary shells throughout life. Now the shelled 

 Gasteropods deviate from bilateral symmetry in the dis- 

 position of both the alimentary system and the reproductive 

 system. The naked Gasteropods, in losing their shells, have 

 lost that immense one-sided development of the alimentary 

 system which fitted them to their shells, and have acquired 



