470 F. W. GAMBLE AND J. H. ASHWORTH. 



as having been formed exclusively by the fusion of sensory 

 areas which at first were simply modified portions of the 

 prostomial epidermis^ and which in all Polychgetes develop in 

 connection with it, while in some the connection is retained 

 throughout life. 



These sensory lobes, however, do not constitute the entire 

 brain even of such forms as Arenicola. In addition there 

 are tracts arising from groups of cells in the brain which 

 traverse the connectives round the oesophagus and extend 

 into the cord, and may even terminate peripherally. Although 

 we know comparatively little of the course of these fibres, 

 the cells from which they spring must be included in any 

 complete explanation of the origin of the brain. Racovitza, 

 however, takes no cognizance of them, and in this respect his 

 explanation of the brain of Polychfetes is incomplete, and 

 his fundamental plan incomplete also. Yet in spite of this 

 the plan is one with which the brain of Arenicola may be 

 usefully compared. 



In the "marina" section there are two pairs of lobes, an- 

 terior and posterior, connected by an intermediate region 

 which partakes histologically of the characters of both. The 

 anterior lobes are specially related to the front end of the 

 prostomium, and the oesophageal connectives are given off 

 from them. The posterior lobes are as closely connected 

 with the ciliated groove or nuchal organ. The intermediate 

 region supplies the upper surface of the prostomium. 



In the two species, A. ecaudata and A. Grubii, this differ- 

 entiation is absent. The brain is commissural in form, and 

 merely unites the ends of the oesophageal connectives. Some- 

 times a posterior expansion underlies the ciliated groove, but 

 as a rule this is supplied by several of the branches, which 

 are rather cerebral nerves than lobes. This simplification of 

 brain structure, as Racovitza has pointed out, corresponds with 

 the simpler character of the prostomium in these species. 



The brain of young specimens (50 to 75 mm. long) of 

 A. marina is pyriform, the broader end being anterior. It 

 might, however, be more correctly described as roughly 



