342 RiilpJ' S- Lillie 



AsiiwoRTii (1598, 1900), who iu their two important papers on the 

 anatomy and afliuities of the Arcnieolidae have added materially to 

 Olir Ivuowledg-e of these organs. The second paper of these authors 

 (1900) contains a comparative account of the anatomy, relations, 

 and vaseulav supply of the uephridia of five species of the genus, 

 viz: — A. marina Linn., A. cristafa Stimpson, A. Claparcdü Levinsen, 

 A. Gruhii Claparède and A. ecaudata Johnston; the authors have 

 also given a summary of our present knowledge of the anatomy 

 and histology of the adult nephridia, and have added a brief account 

 of the condition of the organs in earlier developmental stages^ so 

 far as this has as yet been ascertained. 



Our knowledge of the larvai development of Arenicola has, howe- 

 ver, until recently been too limited to admit of a detailed study of 

 the mode of origin of the nephridia, and of the condition of these 

 Organs at different periods in the life-history. Benham (1893) and 

 Gamule & AsinvoRTH (1S9S, 1900) bave contributed a few obser- 

 vations on the nephridia of certain post-larval stages. Their obser- 

 vatious were made on relatively few specimens which in all cases 

 were well-advanced in development. Accordingly, Ave have had as 

 yet no detailed account of the origin, growth, and histological dift'e- 

 rentiation of the nephridia in Arenicola. One reason for this deficiency 

 seems hitherto to have been the difficulty of obtainiug the material 

 tor such an investigation. In my own studies, however, instead of 

 depending upon the slow and precarious methods of digging and 

 t(ivv-net coUecting:, I have found it more practicable to rear the 

 larvae artiticially; in this manner it has been found possible to 

 secure all the material necessary for a detailed examination of the 

 larvai development from the eavly free-swimming period up to a 

 stage at which the adult eharacteristics are essentially complete. 



The aira of the present study has been to complete so far as 

 possible our knowledge of the origin- and histological ditferentiation 

 of the nephridia, and to determine to what extent the peculiarities 

 of the adult uci)liridiura gain an explanation tlirough the conditions 

 of larvai development. The relations of the nephridia to the body- 

 segmentation and lo the septa, and the nianner of development of 

 the different portions of the organ — nephrostome, glandulär portion 

 and terminal vesicle (whether separately, or by ditferentiation froni 

 a single embryonic rudimeut) — are raatters that have received 

 cspecial attention. There has also been added a brief account 

 of the anatomy and histology of the adult nephridium, draw- 



