THE GENERAL SHAPES OP ANIMALS. 203 



retain rudimentary shells throughout life. Now the shelled 

 Gasteropods deviate from bilateral symmetry in the disposi- 

 tion 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 

 that bilateral symmetry of external figure which fits them 

 for their habits of locomotion; but the reproductive system 

 remains one-sided, because, in respect to it, the relations to 

 external conditions remain one-sided. 



The Cephalopods show us bilaterally-symmetrical external 

 forms along with habits of movement through the water in 

 two-sided attitudes. At the same time, in the radial distri- 

 bution of the arms, enabling one of these creatures to take 

 an all-sided grasp of its prey, we see how readily upon one 

 kind of symmetry there may be partially developed another 

 kind of symmetry, where the relations to conditions favour it. 



§ 252. The Vertebrata illustrate afresh the truths which 

 we have already traced among the Annulosa. Flying through 

 the air, swimming through the water, and running over the 

 earth as vertebrate animals do, in common with annulose 

 animals, they are, in common with annulose animals, different 

 at their anterior and posterior ends, different at their dorsal 

 and ventral surfaces, but alike along their two sides. This 

 single bilateral symmetry remains constant under the ex- 

 tremest modifications of form. Among fish we see it alike 

 in the horizontally-flattened Skate, in the vertically-flattened 

 Bream, in the almost spherical Diodon, and in the greatly- 

 elongated Syngnathus. Among reptiles the Turtle, the Snake, 

 and the Crocodile all display it. And under the countless 

 modifications of structure displayed by birds and mammals, 

 it remains conspicuous. 



A less obvious fact which it concerns us to note among the 

 Vertebrata, parallel to one which we noted among the An- 

 nulosa, is that whereas the lower vertebrate forms deviate 



