182 



MORPHOLOGICAL DEVELOPMENT. 



comnion 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 s^mimetry : 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 uj)per 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 Juhis and the 



Centipede, may be taken as shoT\ang a transition to double 

 bilateral symmetry. Besides being divisible into exactly 

 Bimilar 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 isoj^odous crustaceans, the departure 



from these low types of symmetry is more marked. As 



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

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



