the same part in advance--a form in which, consequently, the original radial symmetry diverges more and more towards bilateral symmetry. § 250. Very definite and comparatively uniform, are the relations between shapes and circumstances among the _Annulosa_: including under that title the _Annelida_ and the _Arthropoda_. The agreements and the disagreements are equally instructive. At one time or other of its life, if not throughout its life, every annulose animal is locomotive; and its temporary or permanent locomotion, being carried on with one end habitually foremost and one surface habitually uppermost, it fulfils those conditions under which bilateral symmetry arises. Accordingly, bilateral symmetry is traceable throughout the whole of this sub-kingdom. Traceable, we must say, because, though it is extremely conspicuous in the immense majority of annulose types, it is to a considerable extent obscured where obscuration is to be expected. The embryos of the _Tubicolæ_, after swimming about a while, settle down and build themselves tubes, from which they protrude their heads; and in them, or in some of them, the bilateral symmetry is disguised by the development of head-appendages in an all-sided manner. The tentacles of _Terebella_ are distributed much in the same way as those of a polype. The breathing organs in _Sabella unispira_, Fig. 260, do not correspond on opposite sides of a median plane. Even here, however, 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. [Illustration: Figs. 260–261.] The community of symmetry among the higher _Annulosa_, has an unobserved significance. That Flies, Beetles, Lobsters, 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, however, it will be seen to have a meaning not to be overlooked; 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. [Illustration: Figs. 263–270.]