NO. 8 THE POLYCHAET — RAW 25 



course of unimaginable time. It is not due merely to the cephaliza- 

 tion of additional anterior segments of the body as many have sup- 

 posed, but is largely due to the advance to the brain of three pairs 

 of segmental body ganglia at three far separated times, and, at the 

 same times also, advance to the head of three segmental pairs of cirri 

 and two segmental pairs of eyes, to become the cephalic sense organs. 

 Incidentally this aggregation caused the brain to incorporate also 

 the hind-brain innervating the nuchal organs, which are therefore 

 just as much parts of the prostomium as are the other cephalic sense 

 organs and the mid-brain. 



Further, this great aggregation of the brain and the cephalic sense 

 organs, which built up the polychaet prostomium, is claimed here 

 as incidental to the evolution of the stomodeum, which again is 

 referred to changes in the mode of feeding. 



MORPHOLOGICAL SIGNIFICANCE OF THE HIND-BRAIN 



The nuchal organs are quite dorsal in position. This is seen very 

 clearly in the Spionidae which in some forms retain them completely ; 

 and it is indicated also by their position in the errant polychaets. But 

 in these it is only in the tetraneural Amphinomidae and Euphro- 

 synidae that extensive development of the nuchal organs occurs. 

 Here development of the caruncle is accompanied both by great 

 enlargement of the hind-brain (presumably by the addition of their 

 ganglia), and by tetraneury ; and again the nuchal organs are quite 

 dorsad of these nerves. These relationships suggest that the ancestor 

 of the polychaet, before the evolution of its brain, stomodeum, and 

 visceral nervous system was characterized by sexneury having : ( i ) 

 a pair of ventral nerve cords (still persisting as the ventral nerve 

 cords of the central nervous system), (2) a pair of lateral or podial 

 nerve cords (still retained only in the tetraneural families above men- 

 tioned), and (3) a pair of dorsal or nuchal nerve cords (still retained 

 by the Spionidae, greatly developed and cephalized by the tetraneural 

 families, and greatly reduced subsequently in other families). 



The mode of evolution of the polychaet detailed above — by three 

 ventral invaginations (two great and one small) — would involve, 

 also, as these took place, the contraction and packing together of the 

 dorsal parts of the ventrally invaginated segments and, therefore, 

 of the nuchal ganglia. The tetraneural families, as they retained 

 their podial longitudinal nerves, took advantage also of this, and 

 have incorporated the nuchal organs into the caruncle, and the nuchal 

 ganglia into the brain. 



