NO. 8 THE POLYCHAET — RAW 3 



The fore-brain. This separates itself from the rest by its position, 

 form, structure, the senses it serves, and the gap above and behind it 

 (through which pass muscles of the mouth-lips, the so-called "palps" 

 of many authors). It is suggested here that this was the primary 

 brain, and as an archicerebrum may well have had a complicated 

 history of its own. It receives the ventral root of the perioesophageal 

 commissure; and gives origin to the two pairs of stomatogastric 

 nerves and to nerves of the mouth-lips. 



The mid-brain. This, the main brain, innervates the prostomial 

 appendages and the eyes. It is clearly subdivided into three successive 

 sections diminishing rearwards — an anterior, middle, and posterior. 

 The anterior receives the dorsal branch of the perioesophageal com- 

 missure, and innervates the anterolateral antennae; the middle in- 

 nervates the single pair of eyes and the posterolateral antennae ; the 

 posterior innervates the median antenna and the posterior eyes when 

 present. Heider, like Pruvot, instituted only two divisions : the last 

 two here instituted forming his posterior division ; but in his figures 

 (loc. cit., figs. 12 and 13 ; see also figs. 3, 4, and 5 accompanying this 

 paper) they are very clearly distinguishable from one another. 



The antennae are comparable with the tentacular cirri of the second 

 buccal segment and with the cirri of the normal body segments. The 

 eyes too may well be homologous with the pigment spots on the body 

 segments above the parapodia and gills. Each of these three sub- 

 divisions of the mid-brain is below claimed to originate in a separate 

 pair of body ganglia added to the brain at different times, and to be 

 the only parts which have homologues in the ventral chain. 



The hind-brain is much smaller, having only a small fraction of the 

 volume of even the smallest (the posterior) division of the mid- 

 brain. It consists of two separate ganglia : these innervate the nuchal 

 organs, which have no counterpart elsewhere in Eunice. 



These three "brains," the fore, mid, and hind, thus constitute three 

 distinct categories, distinct not only in character, but also, it is 

 believed, in their origins and their histories as explained below. 



HISTORY OF OPINION REGARDING THE CONSTITUTION OF 

 THE BRAIN IN POLYCHAETS 



As the theory here submitted differs so fundamentally from all 

 previously presented, a short sketch of the diverse opinions regard- 

 ing the origin and significance of the brain was written, but is with- 

 held in order to shorten this paper. The views of the various authors 

 beginning with Pruvot in 1885 (see list of references, p. 33) are 

 extremely varied; they are summarized in table i. The theory of 



