ANATOMY" OF ARENICOLA ASSIMILIS. 775 



American coutiuent and from South Georgia. Others 

 forming a new variety but agreeing with the type, except in 

 the number of chgetigerous segments, are now recorded from 

 Otago Harbour, the Macquarie Islands, and the Falkland 

 Islands. Post-larval stages of the variety have been obtained 

 off Stanley Harbour (East Falkland) and near Uschuaia. 



VIII. Specific Characters of the Caudate Aroni- 



colidaj. 



Appended is a revised summary of the characters of the 

 caudate Areuicolidse, which clearly shows by what features, 

 both external and internal, A. assimilis may be readily 

 recognised and distinguished from other species with Avhich 

 it is liable to be confused. It cannot be too strongly urged 

 that attention should be directed by systematists chiefly to 

 internal characters in the discrimination of the species of 

 Arenicola. No determination of A. marina, A. clapa- 

 redii, or A. assimilis can be considered complete or 

 entirely trustworthy Avhich relies solely on extei'nal characters. 

 It is impossible to distinguish these three species Avith 

 certainty unless reference be made to the nephridia, 

 oesophageal glands, and otocysts, the two former being of 

 especial use in this connection. 



The characters ^ of the caudate Arenicolidffi may be briefly 

 stated thus : 



A distinct tail present; the parapodia and gills do not 

 extend to the posterior end of the animal. The body is 

 often swollen anteriorly. Cills, pinnate or derivable from 

 the pinnate type, eleven to thirteen pairs, the first (which 

 may be small or even absent) on the seventh or eighth 

 chtutigerous segment. Prostomium consisting of a median 

 and two lateral lobes. Nephrostomes with dorsal lip well 

 provided with flattened, spatulate, ciliated, vascular processes; 



' The following is a revision of a part, of the suiuniary i)ublisheJ by Dr. 

 Gamble and myself (1!)00, j). 510), to which reference may be made for the 

 characters ol the genus and of the ecaudate species. 



