198 



MORPHOLOGICAL DEVELOPMENT. 



spond on opposite sides of a median plane. Even here, how- 

 ever, the body retains its primitive bilateralness ; and it is 

 further to be remarked that this loss of bilateralness in the 

 external appendages, does not occur where the relations to 

 external conditions continue bilateral: witness the Serpula, 

 Fig. 261, which has its respiratory tufts arranged in a two- 



sided way, under the two-sided conditions involved by the 

 habitual position of its tube. 



The community of symmetry among the higher Annulosa, 

 has an unobserved significance. That Flies, Beetles, Lob- 

 sters, Centipedes, Spiders, Mites, have in common the cha- 

 racters, that the end which moves in advance differs from 

 the hinder end, that the upper surface differs from the under 

 surface, and that the two sides are alike, is a truth received 

 as a matter of course. After all that has been said above, 

 however, it will be seen to have a meaning not to be over- 

 looked ; since it supplies a million-fold illustration of the laws 

 which have been set forth. It is needless to give diagrams. 

 Every reader can call to mind the unity indicated. 



While, however, annulose animals repeat so uniformly 

 these traits of structure, there are certain other traits in 

 which they are variously contrasted; and their contrasts 

 have to be here noted, as serving further to build up the 

 general argument. In them we see the stages through which 



