THE GENERAL SHAPES OF ANIMALS. 



199 



bilateral symmetry becomes gradually more marked, as the 

 conditions it responds to become more decided. A 



common Earth-worm may be instanced as a member of 

 this sub-kingdom that is among the least-conspicuously 

 bilateral. Though internally its parts have a two-sided 

 arrangement; and though the positions of its orifices give it 

 an external two-sidedness, at the same time that they estab- 

 lish a difference between the two ends; yet its two-sidedness 

 is not strongly-marked. The form deviates but little from 

 what we have distinguished as triple bilateral symmetry: if 

 the creature is cut across the middle, the head and tail ends 

 are very much alike; if cut in two along its axis by a hori- 

 zontal plane, the under and upper halves are very much 

 alike, externally if not internally ; and if cut in two along its 

 axis by a vertical plane, the two sides are quite alike. 

 Figs. 263 and 264 will make this clear. Such creatures 



as the Julus and the Centipede, may be taken as showing 

 a transition to double bilateral symmetry. Besides being 

 divisible into exactly similar halves by a vertical plane pass- 

 ing through its axis, one of these animals may be bisected 

 transversely into parts that differ only slightly ; but if cut in 



267 



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468 



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two by a horizontal plane passing through its axis, the under 

 and upper halves are decidedly unlike. Figs. 265, 266, 

 exhibit these traits. Among the isopodous crustaceans, 



the departure from these low types of symmetry is more 



