GENERAL ZOOLOGY 



jilM Nephridiostome 



• Septum 

 -Nephridiopore 



Bladder 



Narrow tube 



Fig. 14.9. Nephridium of Limbncus, as seen in optical section; somewhat diagrammatic. Bv 

 following; the convolutions of the tubule from the nephridiostome to the exit through the bodv 

 wall, local differentiations in the characteristics of its walls become obvious. These differences 

 are apparently related to specific differences in function. The extensive capillary networks 

 which cover the nephridium have been removed (cf Fia;. 14.8). (Redrawn from W. B. 

 Benham, 1891, Qiiarlerly Journal oj Microscopical Science, vol. 32.) 



organization of the circulatory system in Lumhncus will be apparent from 

 Figure 14.8. 



The excretory system is comparable with that of Nereis, although in Lurn- 

 bricus the individual nephridia are larger and appear to be more highly 

 specialized (Fig. 14.9). Each is a coiled tubule, bearing at its inner end, 

 exposed to the coelomic fluid, a ciliated funnel, the nephridiostome. The 

 nephridiostome lies in the coelomic cavity of the somite just anterior to that 

 in which the tubular portion is found; the tubule passes through the inter- 

 vening septum before becoming thrown into loops. The distal portion is 

 expanded into a bladder-like swelling, lying near the point at which the 

 tubule pierces the body wall to terminate externally in a nephridiopore. Parts 

 of the tubule, in addition to the nephridiostome, are ciliated; the loops of the 

 tubule are interlaced with capillaries, and the entire mass is enclosed by 

 peritoneum. The morphological relationships are such that two functional 

 interpretations are possible: (1) the open coelomic funnels may be unim- 

 portant in the excretory process, the tubular portions extracting wastes 

 rather from the blood flowing through the nephridial capillary networks; or 

 (2) coelomic fluid may pass into the nephridiostomes and be conducted 

 through the tubules, undergoing modification by the activities of cells lining 

 the tubules — these cells extract from the fluid certain salts which then pass 

 into the circulating blood in the nephridial capillaries. At present, studies 

 in the structure and function of the nephridia of Lumhncus favor the second 

 of these interpretations. The urine which may be collected from the bladder 

 region of the nephridium is more watery than that collected in the upper 

 portions of the tubule, indicating that salts have been withdrawn from the 



408 



