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



of a double origin. It is probably an olfactory organ, and is 

 developed from the posterior region of the prostomium. 



7. The otoliths consist of quartz grains surrounded by a deli- 

 cate chitinoid film, as Ehlers stated. The peculiar commotion 

 observed in otocysts mounted in sea water was not noticed in 

 others examined in coelomic fluid. Hence the motion is 

 probably a result of diffusion currents. 



8. The first pair of nephridia are in process of reduction. 

 In the others the form of the funnel at an early stage is 

 described and figured. In adult examples the terminal 

 portions of the nephridia act as receptacles for the ripe ova 

 or spermatozoa. 



9. The specific gravity of the coelomic fluid varies slightly, 

 but is on the average (including the corpuscles) 1*0288, thus 

 being only very slightly denser than sea water (1'0264). 



10. The general analogies of Arenicola with certain other 

 limnivorous Chsetopods are very striking. With the Sipuuculids 

 the Arenicolidse agree in the chitinous spines tipping the pro- 

 boscis papillae, the buccal papillae, the strong retractors of the 

 '' proboscis," the capacious and largely unsegmented coelom, the 

 general character of the musculature, the thin-walled looped 

 alimentary canal with its ciliated ventral groove, the action of 

 the body-wall in producing waves of coelomic fluid auxiliary to 

 the process of burrowing and defsecation, and lastly, the pig- 

 mented nuchal organ. If we acknowledge the many points 

 of agreement, which have for the most part arisen indepen- 

 dently, between these two distantly related families under 

 similar conditions of life, the true relationship between 

 Arenicola and other genera of Polychsets can only be ascer- 

 tained by exercising the greatest caution in not confusing 

 convergent adaptational characters with true genetic resem- 

 blances. 



