32 PR0CEEDIN08 OF THE NATIONAL MUSEUM. vol. 39. 



KEY TO THE AMERICAN SPECIES OF ARENICOLA. 



1 . Seventeen chsetigerous Begments, 11 pairs of gills 2 



Nineteen or twenty chsetigerous segments, 13 (or 12) pairs of gills 3 



2. Gills large, pinnate; nephridia open on fifth to tenth segments; septal pouches 



large; statocysts closed, each containing one large statolith cristata. 



Gills small, bushy; nephridia open on fourth to ninth segments; septal pouches 

 small; statocysts open, each containing numerous statoliths (sand grains) .gr^aaoHs. 



3. Neuropodia of posterior branchial region long, their grooves extending almost to the 



mid-ventral line; one pair of oesophageal glands; a pair of small septal pouches; 



statocysts open, each containing numerous statoliths (sand grains) marina. 



Neuropodia of posterior branchial region short, forming pads on the sides of the 

 segments, their grooves not nearly reaching to the mid-ventral line; several 

 l^airs of oesophageal glands; no septal pouches 4 



4. Twenty chsetigerous segments; nephridia open on fourth to ninth segments; large 



open statocysts each containing numerous statoliths assimilis. 



Nineteen chsetigerous segments 5 



5. Lateral lobes of prostomium of moderate size^ not folded anteriorly; nephridia 



open on fourth to ninth segments; open statocysts each containing numerous 



statoliths assimilis, var. affinis. 



Lateral lobes of prostomium very large, generally folded at their anterior end, 

 nephridia open on fifth to ninth o segments, no statocysts claparedii. 



In drawing up this key the character of the prostomium has been 

 used as httle as possible, because in many preserved specimens tlie 

 prostomium is either retracted or its lobes have undergone alteration 

 of shape, owing to their having been in contact with the walls of 

 the vessel in wliich the specimens have been kept. 



Considerable care is necessary in discriminating between A. clapa- 

 redii and A. assimilis, var. affinis (section 5 of the key). If the pro- 

 stomial lobes of the specimen under observation are not well pre- 

 served, the only features by means of which the species can be deter- 

 mined are the number of nephridia and the presence or absence of 

 statocysts. Rarely specimens of A. claimredii have an extra nephri- 

 dium opening on the fourth segment, only three such cases have 

 been seen out of about a hundred examined, so that the number of 

 nephridia is not an absolutely reliable character for diagnostic pur- 

 poses. The only way^ of deciding the species in these cases is there- 

 fore to examine, in the dissection which will have been already made 

 to ascertain the number of oesophageal glands and the absence of 

 septal pouches (section 3 of the key), the inner wall of the peristo- 

 mium along the first part of the course of the oesophageal connectives; 

 if statocysts are visible the specimen is one of A. assimilis, var. affinis. 

 Occasionally the statocysts are well imbedded in the musculature, 

 and owing to this and to their small size may escape detection. To 

 definitely prove their absence it is necessary to make serial sections 

 of the anterior end of the specimen, or of that region of the peristomial 

 wall in which one of the statocysts would, if present, be situated. 

 Fortunately, recourse to sections is comparatively seldom necessary 

 for diagnostic purposes. 



o Rarely on fourth to ninth. 



b Since writing this I have found that the form of the neuropodial crotchets 

 usually affords help in distinguishing these two species. 



