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



which they also resemble in structure. The earliest stage 

 in the development of the eye of Nereis, figui'ed by 

 Andrews (1892, pi. xii, fig. 60), is directly comparable with 

 the grade of structure which these organs retain in Areni- 

 cola. The brown or red pigment is very distinctive, and 

 unlike the black pigment with which the epidermis is gene- 

 rally loaded. 



The remaining sensory structures of Arenicola are the 

 sense-cells of the surface of the prostomium, and possibly the 

 notopodial sette. Whether the scattered epidermal sense- 

 cells which have been found in Axiothea and Clymene by 

 Lewis (1898) occur in Arenicola we cannot state, as we 

 have not specially investigated the subject, but, judging 

 from the general similarity between these forms in the 

 nervous systems and other sense-organs, it is very probable 

 that peripheral sense-cells will be found in Arenicola. 



13. Nephridia. 



The nephridia of A. marina have been investigated by 

 several anatomists. The general form and blood-supply 

 of the organ have been recorded by Cosmovici (1880), 

 Cunningham (1887), Benbam (1891, 1893), Kyle (1896), our- 

 selves (1898), and others. The detailed histology of the adult 

 and post-larval nephridium has so far only been desci'ibed in 

 detail by Benliam (1891, 1893). The chief points of interest 

 as yet undecided are the origin of tlie nephrostome, the 

 changes which the whole organ undergoes at different periods, 

 and the relation of the gonad to it. 



The nephridia of A. Claparedii have never been fully 

 described, and the only drawings relating to them are found 

 in Jaquet's paper on the vascular system of Annelids (1886). 



The nephridia of A. cristata have never been investigated, 

 and the descriptions of the organs iuA.Grrubii byClaparede 

 (1868), and in A. ecaudata by Fanvel (1899), merely relate 

 to the general form of the adult organ without reference to its 

 condition in the post-larva or to its finer histological structure. 



The genus Arenicola maybe divided into two sections by 



