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



12. Sense-organs : Otocysts .... 



13. Nephridia ...... 



14. Reproductive Organs .... 



15. Specific Characters and Classification of the Arenicolidse 



16. Affinities of the Arenicolidse 



17. Summary of Results .... 



18. Literature ...... 



PAGE 



500 

 508 

 521 

 528 

 543 

 547 

 554 



1. Introduction. 



Although tliG literature wliicli treats of the anatomy of 

 Arenicola is a large and growing one, there are several struc- 

 tures, such as the nervous system, the nephridia, the " hearts " 

 and " heart-body," which have received little or no attention 

 from anatomists such as de Quatrefages, Grube, Claparede, 

 Pruvot, Racovitza, who have contributed largely to a know- 

 ledge of the comparative morphology of Polychtetes. Embry- 

 ologists have neglected the Arenicolidte even more. Wilson's 

 early paper (1882)^ still remains the only account of the 

 development of an Arenicola (A. cristata) from the egg 

 up to the early post-larval stage, and naturally enough only 

 deals with the external features. In 1893 the post-larval 

 stage of the common lugworm was identified by Dr. Benliam, 

 and though his decision was soon afterwards questioned by 

 Mesnil (1897), Benham's statements have since received full 

 confirmation. It is not our purpose to give in this place a 

 full resume of past or recent work on the Arenicolidae, but 

 merely to point out that both in morphology and embryology 

 there are lacunaB where our present knowledge is scanty or 

 entirely lacking. This statement applies, moreover, to the 

 groups with which it has been suggested, though on some- 

 what slender grounds, that the Arenicolidse are allied, namel}^, 

 the MaldanidfB on the one hand and the Scalibregmidce on 

 the other. For all we know to the contrary, these families 

 may represent three more or less parallel but independent 

 modifications. Their relationship can only be adequately 



1 The dates in brackets form references to the literature quoted at the end 

 of this pnper. 



