LARVAL FORMS. 



313 



The position of the flagellum in Pilidium, and of the supra-oesophageal 

 ganglion, in Mitraria, suggests a different view of the origin of the supra- 

 U3sophageal ganglion from that adopted above. The position of the ganglion 

 in Mitraria corresponds closely with that of the auditory organ in Cteno- 

 phora; and it is not impossible that the two structures may have had a 

 common origin. If this 

 view is correct, we must 

 suppose that the apex of 

 the aboral lobe has be- 

 come the centre of the 

 praeoral field of the Pi- 

 lidium and Trochosphere 

 larval forms 1 a view 

 which fits in very well 

 with their structure 

 (tigs. 226 and 233). The 

 whole of the questions 

 concerning the nervous 

 system are still very ob- 

 scure, and until further 

 facts are brought to light 

 no definite conclusions 

 can be arrived at. 



The 



absence of 

 sense organs on the 

 praeoral lobe of larval 

 Echinodermata, cou- 

 pled with the structure 

 of the nervous system 

 of the adult, points to 

 the conclusion that the 

 adult Echinodermata 

 have retained, and not, 

 as is now usua'ly held, 

 secondarily acquired, 

 their radial symmetry ; 

 and if this is admitted 

 it follows that the ob- 

 vious bilateral sym- 

 metry of Echinoderm 

 larvae is a secondary character. 



The bilateral symmetry of many Coelenterate larvae (the larva of 

 ^Eginopsis, of many Acraspeda, of Actinia, &c.), coupled with the fact 

 that a bilateral symmetry is obviously advantageous to a free-swim- 

 ming form, is sufficient to shew that this supposition is by no 

 means extravagant ; while the presence of only two alimentary 



1 The independent development of the supra-cesophageal ganglion and ventral 

 nerve-cord in Cbatopoda (vide Kleinenberg, Development of Lumbricus trapezoidcs) 

 agrees very satisfactorily with this view. 



FIG. 232. A. PILIDIUM WITH AN ADVANCED NEMER- 



TINE WORM. B. KlPE EMBRYO OF NEMERTES IN THE 



POSITION IT OCCUPIES IN PILIDIUM. ( Both after Biitschli.) 



OR. oasophagus ; st. stomach ; i. intestine ; pr. pro- 

 boscis; Ip. lateral pit (cephalic sack); an. amniou; n. 

 nervous system. 



