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



nerve tracts, while the comparatively small size attained by 

 A. Claparedii may account for the lack of demarcation of 

 regions which is such a noteworthy character in the brain of 

 large specimens of A. marina. ' 



These considerations raise the question of the real nature 

 of the so-called brain in these forms. No doubt it is essen- 

 tially a centre for afferent impulses received from the sensory 

 prostominm and prostomial sense-organs, as well as from the 

 nerves of the buccal papillte, of the lining of the buccal mass 

 and possibly from the gastric plexus further back still. But 

 in addition to this, at least one pair (and probably more) of 

 efferent centres^ are found in it, and it is probable that from 

 these efferent impulses travel doAvn the connectives by certain 

 tracts to the nerve-cord. Compared with the arthropod 

 brain, however, the development of these tracts binding 

 the different parts of the brain together, and connecting the 

 brain through the cord with peripheral endings, must be con- 

 sidered in Arenicola and Polycheetes generally as but slight. 

 Still there are indications of them, and future investigation 

 will have to discover bow far the impulses derived from the 

 prostomiura and buccal sense-endings are followed by effer- 

 ent impulses affecting the muscular glandular tissues ; what 

 is the course taken by the reflex action, and also what 

 reflexes are possible in the absence, or apart from the control, 

 of the brain. The few physiological observations which 

 have hitherto been made on Polychfetes .scarcely afford 

 ground for conclusions on this subject. In Crustacea, how- 

 evei*, some recent and very suggestive I'esults have been ob- 

 tained by Bethe, which have a distinct bearing on the problem 

 of the significance of neuropile in initiating efferent impulses. 

 Since it is in the brain of Arenicola, and also, according to 

 Hamaker (1898), in that of Nereis, that neuropile is chiefly 

 present, Bethe's experiments may throw some light on the 

 function of the Polychsete brain. 



Bethe's experiments (1898i) were carried out on crabs. 



' The paired masses of large pyriform ganglion cells placed near the middle 

 line of the anterior cerebral lobe (p. 475). 



