LARVAL FORMS. 379 



above-mentioned Nemertines and Peripatus, that the commissure 

 connecting the two nerve-cords behind is placed on the dorsal 

 side of the intestine. As is at once obvious, by referring to the 

 diagram (fig. 231 B), this is the position this commissure ought, 

 undoubtedly, to occupy if derived from part of a nerve-ring which 

 originally followed more or less closely the ciliated edge of the 

 body of the supposed radiate ancestor. 



The fact of this arrangement of the nervous system being 

 found in so primitive a type as the Nemertines tends to establish 

 the views for which I am arguing ; the absence or imperfect 

 development of the two longitudinal cords in Turbellarians may 

 very probably be due to the posterior part of the nerve-ring 

 having atrophied in this group. 



It is by no means certain that this arrangement of the nervous 

 system in some Mollusca and in Peripatus is primitive, though it 

 may be so. 



In the larvae of the Turbellaria the development of sense organs in the 

 praeoral region is very clear (fig. 222 B) ; but this is by no means so obvious 

 in the case of the true Pilidium. There is in Pilidium (fig. 232 A) a thicken- 

 ing of epiblast at the summit of the dorsal dome, which might seem, from 

 the analogy of Mitraria, etc. (fig. 233), to correspond to the thickening of the 

 prasoral lobe, which gives rise to the supra-cesophageal ganglion ; but, as a 

 matter of fact, this part of the larva does not apparently enter into the 

 formation of the young Nemertine (fig. 232). The peculiar metamorphosis, 

 which takes place in the development of the Nemertine out of the Pilidium 1 , 

 may, perhaps, eventually supply an explanation of this fact ; but at present 

 it remains as a still unsolved difficulty. 



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

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

 cesophageal 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 become the centre of the prseoral field of the Pilidium 

 and Trochosphere larval forms 2 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. 



1 Vide Vol. II. p. 204. 



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

 nerve-cord in Chsetopoda (vide Kleinenberg, Development of Lumbricus trapezoides) 

 agrees very satisfactorily with this view. 



