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many Nemertines^ (the Euopla and Pelagonemertes), in 
Peripatus,^ and in primitive molluscan types (Chiton, Fis- 
surella, &c.). From the lateral parts of this ring it is easy 
to derive the ventral cord of the Chsetopoda and Arthro- 
poda. It is especially deserving of notice, in connection 
with the nervous system of 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. 18), 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 radial 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 Tur- 
bellarians 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 primi- 
tive, though very probably it may be so. 
In the larvse of the Turbellaria^the development of sense 
organs in the praeoral region is very clear (fig. 9 b); but this 
is by no means so clear in the case of the true Pilidium. 
There is in Pilidium (fig. 19 a) a thickening of epiblast 
at the summit of the dorsal dome, which might seem, 
from the analogy of Mitraria, &c. (fig. 20), to correspond to 
the thickening of the prseoral lobe, which gives rise to the 
supra-oesophageal ganglion ; but, as a matter of fact, this 
part of the larva does not apparently enter into the formation 
of the young Nemertine (fig. 19). The peculiar metamor- 
phosis which takes place in the development of the Nemer- 
tine out of the Pilidium,^ may, perhaps, eventually supply 
an explanation of this fact ; but at present it remains as a 
not yet explained difficulty. 
The position of the fiagellum in Pilidium, and of the supra- 
oesophageal ganglion in Mitraria, suggests a different view of 
the origin of the supra-oesophageal ganglion to that adopted 
above. The position of the ganglion in Mitraria corresponds 
^ * Vide Hubrecht, ‘ Zur Anat. and Phys. d. Nerven System d. Nemer- 
tinen Kon. Akad.,’ Wiss., Amsterdam; and “Researches on the Nervous 
System of Nemertines,” ‘ Journ. of Micr. Science,’ 1880. 
’ Vide Seif, “ On some points in the Anat. of Peripatus capensis'^ ‘ Quart. 
Journ. of Micr. Science,’ vol. xix, 1879. 
’ Vide ‘ A Treatise on Comparative Embryology,’ vol. i, p. 169. 
