360 Kalpli S. Lillie 



blood-vesscl and formcd by a proliferation of the peritoneal covering 

 of the latter, in a manner later to l)e described. The position of 

 the anterior margin of the nephrostome corresponds to the bouudary 

 between two successive somites; in early larvai life the nephrostome 

 actually perforates the septum and opeus in the typical manner 

 iato the body cavity beyond. In adult life no septa are present 

 in the nephridial regiou; these structures are, however, represented 

 by the segmentai blood-vessels which pass between the subintestinal 

 vessel and the lateral longitudinal vessels. These vessels, with which 

 the nephrostomial blood-vessels are directly continuous, are origi- 

 nally formed in the intersegmental septa, as already described; and 

 in this sense eaeh nephrostome may be considered to open throughout 

 life info the next anterior somite. The external opening of the 

 nopbridium is at the posterior extremity of the somite, dorsal and 

 a little posterior to the row of ventral hooked setae. 



In its naturai position, each nephridium is situated immediately 

 within the dorsal Insertion of the transverse muscular bands, by 

 which it is partially isolated from the general body-cavity. When 

 these muscles are cut and turned back, the nephridia are freely 

 exposcd to view (Piate 22, Fig. 1). Each is theu seen as an elongated 

 tubulär organ with brown walls, terminated posteriorly at the level 

 of the setae by the contractile vesicle, and anteriorly by the larga 

 and characteristic nephrostome, conspicuous from the bright red 

 processes of its dorsal lip. A slip of transverse muscle is attached 

 to the anterior portion of the dorsal lip and in life binds the nephro- 

 stome down in such a manner that its opening is directed inward. 

 Figure 3 represents a nephridium seen in its naturai position from 

 above, after the removal of ali the transverse musculature, except 

 a portion of the attached slip. 



The nephrostome is of peculiar and complex structure and 

 presenta certain pcculiarities that a])parently bave not hitlierto been 

 observed in the other species of Arenicola. Figure 2, which repre- 

 sents an enlarged view of tlic first nephridium of the right side, 

 shows the api)earance of the neplirostome after its attachments bave 

 been removed, and its aperture has been freely exposed to view 

 and spread open. It will be observed that a well defined line of 

 division separates the light-colored non-pigmented nephrostome from 

 the deep brown glandulär jìortion of the organ, a division correspon- 

 ding to a marked dilference in the histological character of the 

 liniug epithelium of the two regions. 



