84 Arenicolidae 



4. Twenty chaetiferous segments ; nephridia opening on the 



fourth to the ninth segments; statocysts large, each 

 with a tube leading to the exterior and with numerous 

 statoliths assimilis, p. 123. 



Nineteen chaetiferous segments 5. 



5. Lateral lobes of prostomium of moderate size, not dilated 



or folded anteriorly ; nephridia opening on the fourth (or 

 fifth 1 ) to the ninth segments ; statocysts present, each 

 with a tube leading to the exterior and with numerous 



statoliths assimilis var. 



Lateral lobes of prostomium large or very large, generally ^ffims, p. 124. 

 folded at their anterior end ; nephridia opening on 

 the fifth 2 to the ninth segments; statocysts absent; 

 the post-rostral region of the neuropodial crotchets 

 is more dilated and convex than in the preceding 



species pusilla, p. 114. 



(=2 claparedii). 



6. Gills large, pinnate ; nephridia opening on the fifth to the 



tenth segments; septal pouches large; statocysts 



closed, each containing one large secreted statolith cristata, p. 105. 



Gills small, bushy ; nephridia opening on the fourth to the 

 ninth segments ; septal pouches small ; statocysts each 

 with tube to the exterior and with numerous statoliths 

 (sand-grains) glacialis, p. 111. 



It is advisable not to depend on external characters alone when the 

 species marina, assimilis and pusilla are concerned. An incision 

 should be made along the mid-dorsal line of the specimen, extending 

 from the eleventh segment nearly to the prostomium, so as to 

 permit examination of all the important organs. Especial care is 

 necessary in discriminating A. pusilla and A. assimilis var. affinis. 

 If the prostomial lobes are well preserved they offer most valuable 

 help in diagnosis, but if their preservation is defective reference 

 should be made in turn to the statocysts, crotchets and nephridia. 

 As the statocysts are more or less imbedded in the muscle and 

 connective tissue, careful search is necessary, or they may escape 

 observation. 



The Ecaudate Section of the Genus ARENICOLA. 



The characters of the ecaudate section (Arenicolides Mesnil 3 ) 

 may be stated thus : The parapodia, and generally also the gills, 

 extend practically to the posterior end of the worm ; there is not an 

 elongate " tail," but behind the last chaetiferous annulus there is a 

 terminal conical portion, consisting of not more than about six 



1 In South African specimens of A. assimilis var. a finis, 



2 Barely on the fourth to the ninth (see p. 118), 



3 See p. 31. 



