364 Ralph S. Lillie 



publislied drawÌDgs of nepliridia of young- specimens of Ä. marina, 

 respectively 30 and 44 mm. in leng-th (Figs. 16, 18), in wbich the 

 dorsal processes are just beginniug thcir formation in a manner that 

 correspouds substantially with the above description. In Ä. crisfata 

 it is i)robable tbat thcy arise at a corresponding stage and in the 

 same manner. It is iuteresting to note that in certain other species, 

 uotably Ä. Grubii and Ä. ecaudata, the processes, while evidently 

 hearing the same relation to the nephrostomial blood-vessel, are 

 fewer in number and simpler in structure, remaining apparently in 

 a less ditìerentiated condition throughout life (Gamble & Ashworth 

 1900). 



Ali indications therefore point to the conclusion that the processes 

 represent simply an elaborate and regulär system of folds of the 

 nephrostomial epithelium. The correctness of this view is further 

 indicated by the identity in structure of the ciliated cells of the 

 processes and of the other portions of the nephrostome. 



By the formation of the processes of the dorsal lip, the ciliated 

 surface of the nephrostome is enormously exteuded and its efficiency 

 as an organ for removing suspended solid par ti des from the 

 coelomic fluid is correspondingly increased. The cilia which cover 

 the processes are incessantly and vigorously active (a condition 

 presumably favored by the abundant vascular supply), and tend to 

 sweep ali suspended solid particles into the interior of the nephri- 

 dium. In liviug Arenicolae it is found that the interior of the 

 nephrostome and the interstices between the processes are occupied 

 by a mass of loosely granular substance evidently collected in this 

 way. On mìcroscopical examination the mass is found to consist 

 largely of broken-down cells coutaiuing excretory granulcs similar 

 to those found in the chloragogeu cells coating the ventral blood- 

 vessel. It appears probable, in fact, that the chloragogeu cells 

 undergo a continuai proccss of disintegration and that the solid 

 excretory products thus set free in the coelom are swept into the 

 interior of the nephridia by the nephrostomial cilia and so conveyed 

 to the exterior. The broken-down chloragogeu cells are possibly 

 replaced by wandering cells or, more specifically , excretophores, 

 which are chemotropically attracted to the ventral blood-vessel and 

 while there extract from the blood the excretory matters; the latter 

 beiug deposited in a solid form in the interior of the cells aud 

 eventually set free in the coelom to he removed to the exterior by 

 the uei)hridia. Urea and other soluble waste-products seem, on the 



