12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. Ill 



to the pharyngeal sacs. It contains the "lower jaw" plates, here 

 interpreted as representing another pair of appendages added to the 

 mouth armature ; and it is bounded by a new, the present, lower lip 



Each of these pharyngeal invaginations has produced a two- fold 

 sac, each sac being partly subdivided by the pair of pads, which can 

 be likened to parapodia, on which the pair or pairs of denticles are 

 situated. Besides the rifts between these pads, the parting between 

 the upper jaw sacs and the lower jaw sacs, as also that between the 

 upper jaw sac and the oesophagus, are markedly bifid. This suggests 

 that with the evolution of each new sac the previous lower lip became 

 bifid owing to the necessity for through-communication through the 

 sacs to the oesophagus. 



The visceral nervous system and its distribution, as suggested 

 above, are explicable as direct results of the invaginations, which 

 not only involved ordinary ectoderm and pairs of appendages, but 

 also the "central" nervous system, which was still continuous with 

 the ectoderm. It may not be possible to assign to each invagination 

 its exact contribution to the visceral nervous system; on the other 

 hand this is clearly subdivisible into an oesophageal and a pharyngeal 

 section. 



At this early stage, too, the main nerve cords from the brain were 

 in a more primitive position than now obtains; for, judging from the 

 parts of the visceral system both on the oesophagus and the jaw sac, 

 they were ventrolateral, not midventral, in position ; and, that this 

 was still the case after the evolution of the polychaet, is suggested 

 by the far-separated ganglion chains of Serpulae and of some 

 arthropods. 



Seeing that each invagination was rearward in direction, its efifect 

 on the nerve chains, when it involved them, was to pull them back 

 into a pair of loops open in front; and their anterior connections, i.e., 

 with the brain, were enormously stretched, in contrast with their 

 posterior connection, i.e., with the continuing nerve cords. An efifect 

 of this is to be seen in the fact that the oesophageal visceral nerve 

 cords from the fore-brain are quite free from the hypodermis, until, 

 in the supra-oesophageal visceral ganglion, they reach the oesophagus. 

 The stretch has pulled them free, as it has also the fore-brain. 



The open loop in the nerve cords, after each invagination that in- 

 volved them, seems to have been closed by the advance of the fore- 

 most of the unaffected ganglia to the brain. Such a change might be 

 aided by the existence of a hypodermal nerve net. 



It is not to be expected that the whole history will now be trace- 



