ANNELIDA 343 



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

 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 from a comparison 

 of the mode of locomotion of, the two forms. The Medusa 

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

 motor apparatus, depending upon muscular action. On the 

 other hand, the Trochophore, with its trochal organ operated 

 by ciliary motion, represents a much more primitive loco- 



^motor apparatus, directly comparable in it^ mode of action to 



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



hich have been advanced against the derivation of the 



fCtenophora from Medusse). A chief difficulty in the 

 derivation under discussion is found in the presence of the 



I central nervous system at the apical region, where important 



Forgans are never developed in the Medusas. We should then 



[Jiave to look upon the nerve-ring of the Trochophore as the 

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

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

 iresent 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 larvae), whereas the ring-nerve, it is to be assumed, 

 has been formed in connection with the development of the 



I 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 



