THE GENERAL SHAPES OF ANIMALS. 185 



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

 mdetermiiiateness of form and the variety of form are 

 accounted for. 



Among the Gasteropods, modifications of a more definite 

 kind occur. &quot; In all Mollusks,&quot; says Professor Huxley, 

 &quot; 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.&quot; 

 In some Gasteropods, as the Chiton, this bilateral sym 

 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. &quot; This asymmetrical over- development never affects 

 the head or foot of the mollusk : &quot; 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 organiza 

 tion as are not exposed to bilaterally- symmetrical conditions. 

 There is reason to believe that the naked Gasteropods are 

 descended from Gasteropods that liad 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 

 pystem which fitted them to their shells, and have acquired 



