182 



MORPHOLOGICAL DEVELOPMENT. 



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 ; 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 passing through the axis, 

 one of these animals may be bisected transversely into parts 

 that differ only slightly ; but if cut in two by a horizontal 

 plane passing through the 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 marked. As 



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shown in Figs. 267 and 268, the contrast between the upper 

 and under parts is greater, and the head and tail ends differ 



