186 MORPHOLOGICAL DEVELOPMENT 



that bilateral symmetry of external figure which, fits tlicm 

 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, which are interpretable as higher de- 

 velopments on the Gfasteropod type, show us bilaterally- sym- 

 metrical external forms along with habits of movement through 

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

 distribution 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. Plying 

 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 ani- 

 mals, 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 extremest 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 

 Annulosa, is that whereas the lower vertebrate forms deviate 

 but little from triple bilateral symmetry, the deviation be- 

 comes great as we ascend. Figs. 273 and 274 show how, 

 besides being divisible into similar halves by a vertical plane 

 passing through its axis, a Fish is divisible into halves that 

 are not very dissimilar by- a horizontal plane passing through 



