ANNELIDA 343 



behind the ciliated band must be considered as the sub-ura- 

 brella, made to bulge downward. A more careful considera^ 

 tion, however, offers considerable difficulties to a derivation 

 of this kind. Even if we disregard the fact that the Medusa 

 represents the most divergent and most highly developed 

 form of the Cnidaria type, and that forms which are highly 

 developed in one direction ordinarily do not become points 

 of departure for new developmental series, still the difficulty 

 of the derivation suggested is evident fi'om a comparison 

 of the mode of locomotion of the t\vo forms. The Medusa 

 moves by means of oar-like strokes of a complicated loco^ 

 motor apparatus, depending iipon uiuscular action. On the 

 other hand, the Trochophore, with its trochal organ operated 

 by ciliary motion, represents a much more prim.itive loco- 

 motor apparatus, directly comparable in its mode of action to 

 the ciliated planula (comp. p. 154, et seq., the grounds 

 which have been advanced against the derivation of the 

 Ctenophora from Medusae). A chief difficulty in the 

 derivation under discussion is found in the presence of the 

 central nervous system at the apical region, where important 

 organs are never developed in the Meduste. We should then 

 have to look upon the nerve-ring of the Trochophore as the 

 chief part of the central nervous system, and the apical plate 

 as a subsequently acquired secondary part of it ; but in the 

 present state of our knowledge we are not justified in this. 

 We recognize that the two parts of the nervous system be- 

 long together, and have probably been developed in close 

 relation with the locomotor apparatus, as regulators of the 

 movements. Thus perhaps the apical plate in its earliest 

 origin is to be traced back to a tuft of cilia functioning as 

 a rudder (such as is met with at the apical pole of many 

 Actinian larvaa), whereas the ring-nerve, it is to be assumed, 

 has been formed in connection with the development of the 

 trochal organ, both of them as localizations of a system of 

 nervous internuncial fibres, distributed under the entire 

 ectoderm. It might be mentioned here that, in addition to 

 the apical plate, a nerve-ring is also met with in the Pili- 

 dium. 



We have above adduced the difficulties which, according 



