180 MORPHOLOGICAL DEVELOPMENT. 



radial structure is constant and complete ; it do3s not follow 

 that the above reasonings are erroneous. On the contrary 

 the derivation of these radially-symmetrical forms from forms 

 not radially-symmetrical, would show how entirely the 

 structure of the organism is moulded by the distribution of 

 forces to which its mode of life exposes it. 



The remaining Aiinuloida, most of them parasitic, must 

 be passed over. Living within the bodies of other creatures, 

 they have their forms determined by conditions that are too 

 obscure to be satisfactorily dealt with. 



§ 250. Very definite and comparatively uniform, are the 

 relations between shapes and circumstances among the 

 Annulosa — including under that title the Annelida and the 

 Articulata. 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 through- 

 out 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 consider- 

 able extent obscured where obscuration is to be expected. 

 The embryos of the Tuhicolce, after swimming about 

 awhile, 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 develop- 

 ment 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 

 iiwspira, Fig. 260, do not correspond on opposite sides of a 

 median plane. Even here, however, the body retains its 

 imiaitive bilateralness : and it is further to be remarked that 



