312 PRIM in VE TYPE OF NERVOUS SYSTEM. 



with which radially arranged sense organs may have been connected ; 

 and that in the Echinodermata this form of nervous system has been 

 retained, while in the other types it has been modified. Its anterior 

 part may have given rise to supra-oesophageal ganglia and organs 

 of vision ; these being developed on the assumption of a bilaterally 

 symmetrical form, and the consequent necessity arising for the sense 

 organs to be situated at the anterior end of the body. If this 

 view is correct, the question presents itself as to how far the posterior 

 part of the nervous system of the Bilateralia can be regarded as 

 derived from the primitive radiate ring. 



A circumoral nerve-ring, if longitudinally extended, might give 

 rise to a pair of nerve-cords united in front and behind — exactly 

 such a nervous system, in fact, as is present in many Nemertines* (the 

 Enopla and Pelagonemertes), in Peripatus^, and in primitive molluscan 

 types (Chiton, Fissurella, etc.). From the lateral parts of this ring 

 it would be easy to derive the ventral cord of the Chtetopoda and 

 Arthropoda. It is especially deserving of notice in connection with 

 the nervous system of the 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 Moll u sea and in Peripatus is primitive, though it 

 may be so. 



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

 prseoral 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 

 thickening 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 prseoral lobe, which gives rise to the stipra-cesophageal ganglion; but, 

 as a matter of fact, this pnrt of the larva does not apparently enter into 

 the formation of the young Nemertine (Hg. 232). The peculiar metamor- 

 phosis, which takes place in the development of the Nemertine out of the 

 Pilidium^ may, perhaps, eventually supply an explanation of this fact; but 

 at present it remains as a still unsolved difficulty. 



1 Vide Hubrecht, " Zur Anat. und Phys. d. Nerven- System, d. Nemertinen, " ^on. 

 Akad. Wiss., Amsterdam; and "Eesearches on the Nervous System of Nemertines," 

 Quart. Journ. of Micr. Science, 1880. 



^ Vide F. M. Balfour, "On some points in the Anat. of Peripatus capensis," Quart. 

 Journ. of Micr. Science, Vol. xix. 1879. 



3 Vide Vol. I. p. 169. 



