26 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. Ill 



MORPHOLOGICAL SIGNIFICANCE AND RELATIONSHIPS 

 OF THE PERISTOME 



On our theory the peristome is not a somite but marks the site of 

 a succession of breaks, causing a gap in an originally continuous 

 sequence of ganglia, the remainder of which form the ventral nerve 

 cord. Further, in the course of evolution the peristome has been 

 extensively exchanged three times over. The missing members are 

 believed to be partly distributed along the visceral nervous system, 

 and partly coalesced with the brain. The magnitude of this "gap" 

 furnishes a complete explanation of the hitherto mysterious fact — 

 that the nervous system arises from two distinct rudiments, one 

 giving rise to the brain, the other to the ventral chain — a fact which 

 has been noted by numerous workers from Salensky (1882) and 

 Goette (1882) onward. The visceral ganglia are thus brought into 

 the succession of those of the central nervous system, in opposition 

 to the generally accepted view of their separate origin. With more 

 detailed knowledge of the visceral nervous system it may be possible 

 to determine in large degree the succession of the various ganglia 

 which can be recognized. 



In the brain, besides the fore-brain and hind-brain we have an 

 association of three pairs of comparable ganglia, sending out com- 

 parable pairs of nerves, and innervating comparable organs; and 

 according to this theory they were added to the brain at three different 

 times. Further, they were selected by nature in the course of evolu- 

 tion from a considerable length of the primitive ventrolateral nerve 

 cords, including probably well over a dozen pairs of ganglia. The 

 order, too, of the upbuild of the brain on this theory is quite different 

 from that on any other. The contrast between this plan and the others 

 that have been advanced could hardly be greater, whether we com- 

 pare it with that of Pruvot, or Racovitza, Nilsson, Lameere, or 

 Hanstrom. The reason for this is that it connects the three systems — 

 the brain, the visceral nervous system, and the stomodeum. It is 

 curious that the order of the elements of the mid-brain happens to 

 be the same as on Lameere's theory ; but the significance is entirely 

 different. It may be remarked that there is much to be said in favor 

 of Lameere's theory as a mode of origin of the annelid ; not, however, 

 of the polychaet but of its distant ancestor. 



Should our theory prove correct, Racovitza will have builded wiser 

 than he knew, though quite other than what he meant ; for the primal 

 brain was his fore-brain, to this was added his mid-brain, and to this 

 the hind-brain. 



But the evolution of the head and brain envisaged here is entirely 



