THE GENERAL SHAPES OF ANIMALS. 



181 



tills 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 

 characters, 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, how- 

 ever, it will be seen to have a meaning not to be overlooked ; 

 since it supplies a million-fold illustration of the laws that 

 have been se* forth. It is needless to give diagrams. Every 

 reader can call to mind the unity indicated. 



TVTiile, 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 bilateral symmetry becomes gradually more marked, as 

 the conditions it responds to become more decided. A 



