NO. 8 THE POLYCHAET — RAW 5 



the present writer, expressed cryptically on the last line, is developed 

 below. 



SIGNIFICANCE OF THE DIVISIONS OF THE BRAIN IN 

 PRIMITIVE POLYCHAETS 



The fore-brain. Both the morphological position and the importance 

 assigned to the fore-brain vary greatly. By Pruvot and by Nilsson 

 it was regarded as the first of three head segments (though the other 

 two of these supposed segments given by the one are different from 

 those given by the other). Hanstrom, on the other hand, is in the 

 opposite camp, regarding it as the sole addition to the brain. To 

 Hatschek, Racovitza, Binard and Jeener, and Soderstrom it was 

 perhaps of little significance. 



But in primitive polychaets, as already remarked, it is separated 

 from the mid-brain by an extensive muscle gap and may exhibit 

 large size as in Eunice and Euphrosyne. In polychaets generally it 

 gives issue to a pair of stomatogastric nerves, and in primitive forms 

 it innervates the mouth-lips which evidently preceded antennal palps 

 in the palpal function. This strongly suggests its great antiquity 

 as an element of the brain. If, however, the stomatogastric lobes of 

 the Amphinomidae and Euphrosynidae were the exact equivalent 

 of the fore-brain of Eunice, as Gustafson seems to claim, there is a 

 significant difference between them; for the ventral root of the 

 perioesophageal connective enters the fore-brain in Eunice, but does 

 not enter the stomatogastric lobes in these other families. This great 

 discordance does not seem to have been remarked by Gustafson, and 

 needs explanation. The equivalent in Amphinomidae of the fore- 

 brain must also include the part which receives the ventral root of 

 the perioesophageal connectives. 



The characters and relationships of the fore-brain, therefore, quite 

 justify one in assigning it to a different category from that of the 

 adjoining mid-brain; and the fact that in the most primitive poly- 

 chaets it innervates the organs which function as palps, suggests that 

 it is the primal brain. 



The hind-brain. The significance of the hind-brain is clarified by 

 the discoveries of Soderstrom. It again falls in quite a different cate- 

 gory from that of the mid-brain, as is indicated by its innervation of 

 the nuchal organs alone. The fact, too, that in the Amphinomidae 

 and Euphrosynidae these organs extend over several segments of the 

 body, and that the hind-brain in them is so developed as to suggest 

 the incorporation in it of several pairs of nuchal ganglia, suggests 

 that the hind-brain is the latest addition to the brain. This is also 



