The position of the flagellum in Pilidium, and of the supraoesophageal ganglion in Mitraria, suggests a different view of the origin of the supraoesophageal ganglion from that adopted above. The position of the ganglion in Mitraria corresponds closely with that of the auditory organ in Ctenophora; 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 become the centre of the præoral field of the Pilidium and Trochosphere larval forms[146]--a view which fits in very well with their structure (figs. 226 and 233). The whole of the questions concerning the nervous system are still very obscure, and until further facts are brought to light no definite conclusions can be arrived at. [146] The independent development of the supraoesophageal ganglion and ventral nerve-cord in Chætopoda (_vide_ Kleinenberg, _Development of Lumbricus trapezoides_) agrees very satisfactorily with this view. The absence of sense organs on the præoral lobe of larval Echinodermata, coupled 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 usually held, secondarily acquired, their radial symmetry; and if this is admitted it follows that the obvious bilateral symmetry of Echinoderm larvæ is a secondary character. [FIG. 232. A. PILIDIUM WITH AN ADVANCED NEMERTINE WORM. B. RIPE EMBRYO OF NEMERTES IN THE POSITION IT OCCUPIES IN PILIDIUM. (Both after Bütschli.) _oe._ oesophagus; _st._ stomach; _i._ intestine; _pr._ proboscis; _lp._ lateral pit (cephalic sack); _an._ amnion; _n._ nervous system.] The bilateral symmetry of many Coelenterate larvæ (the larva of Æginopsis, of many Acraspeda, of Actinia, &c.), coupled with the fact that a bilateral symmetry is obviously advantageous to a free-swimming form, is sufficient to shew that this supposition is by no means extravagant; while the presence of only two alimentary diverticula in Echinoderm larvæ is quite in accord with the presence of a single pair of perigastric chambers in the early larva of Actinia, though it must be admitted that the derivation of the water-vascular system from the left diverticulum is not easy to understand on this view.