THE family Arenicolidae, though small in the number of its genera 

 and species, holds an important place in the literature of Polychaeta. 

 One of the members of this family Arenicola marina, the common 

 lugworm is the most abundant and most readily accessible Poly- 

 chaete of northern and western Europe. Probably no other marine 

 worm has been so frequently observed, collected and dissected, and, 

 consequently, the records and descriptions of which it is the subject 

 form a very extensive series. Twenty years ago the anatomy of the 

 other species of Arenicola was almost unknown ; the accounts of the 

 internal organs, given in descriptions of the genus Arenicola, referred 

 only to A. marina, j,nd it seemed to be taken for granted that all the 

 other species agreed in structure with this. Several of the internal 

 organs, notably the statocysts, oesophageal caeca, nephridia and septal 

 pouches have, however, been found to exhibit well-marked differences, 

 in form or number, in the various species, and to afford considerable 

 help in systematic work. In preparing the diagnoses, I have made 

 full use of the internal characters, the value of which will be 

 especially appreciated in those cases where it is necessary to deter- 

 mine defectively preserved or incomplete examples, which, as 

 experience has shown, can seldom be diagnosed safely by examination 

 of their few and imperfect external features. By means of the keys 

 and diagnoses provided, the identification of the members of the 

 family Arenicolidae will, it is hoped, be accomplished with certainty 

 and with comparative ease. 



Questions of synonymy have received careful consideration and 

 full treatment in the text. I have examined all the extant types of 

 the species of Arenicola, and, in the case of those species the types 

 of which are no longer preserved, I have analysed the published 

 descriptions, and have compared them, with the long series of speci- 

 mens at my disposal, with the result that I can confidently state 

 my conviction that all the known forms fall into the eight species 

 described in this Catalogue. 



In addition to the British Museum Collection, the entire 



