14 CCELOM AND EXCRETORY ORGANS 



Teleostean Fishes). Such funnel-like coelomostomes are developed 

 on the coelomoducts of the ovarian and spermarian segments of 

 the Earthworms and in many Chsetopoda, also in Vertebrata 

 (peritoneal funnels of the reno-genital system) and in some Mollusca 

 (reno-pericardiac funnels). The coelomoducts and the gonocoels, 

 of which they are a part, frequently acquire a renal excretory 

 function, and may retain both the function of genital conduits and 

 of renal organs, or may, where several pairs are present (meta- 

 merised or segmented animals), subserve the one function in some 

 segments of the body, and the other function in other segments. 

 Again in some Mollusca (Gastropoda) it appears that the renal 

 function may be developed by the ccelomoduct and gonocoel 

 of the right side, and the oviducal or seminiducal function by 

 those of the left side of the body. This very general assumption 

 by some or all of the primary gonocoels and coelomoducts of renal 

 excretory functions has led to a confusion of these structures 

 with the primitive ectodermal excretory tubes, which are best 

 distinguished by the name " nephridia." The typical "nephridium" 

 to which the name was originally given (see Lankester, Quart. 

 Journ. Micr. Sci. 1880), is the so-called "segmental organ" of the 

 Earthworm. This occurs as a pair of minute coiled tubes in each 

 segment of the worm's body. Nephridia are distinguished by 

 their independent origin, each from a single superficially placed 

 cell which often is seen to be derived from ectoderm, and probably 

 must be traced to that layer even when it appears as part of the 

 mesoblast. They are also distinguished by their structure, which 

 is primarily that of a number of perforated or drain-pipe cells, 

 placed as it were end to end. It is not necessary to suppose that 

 this uniserial cellular structure is absolutely diagnostic of nephridia, 

 but it seems not improbable that it is so. 



Instead of being, as was supposed, the common origin of 

 the renal organs of all the Coelomocoela, it now appears (see 

 especially G-oodrich's series of memoirs in the Quart. Journ. Micr. 

 Sci. 1897-1900) that the nephridia are a primitive form of 

 excretory organ which have been replaced in the higher groups 

 of Coelomocoela by uropoetic coelomoducts. True nephridia are 

 only found in the Platyhelmia, Nemertina, Rotifera, Chsetopoda, 

 and in embryonic Mollusca (primitive kidneys of Pulmonata and 

 Lamellibranchia). 



The tubular organs, whether renal or genital in function, which 

 have been identified of late years (by myself and others) with 

 nephridia, such as the kidneys of Mollusca, the segmental excretory 

 ducts of Peripatus, the genital and excretory ducts of Arthropods, 

 and the peritoneal funnels and tubules of Vertebrata, are all 

 coelomoducts and not nephridia in the true sense of that word. 

 A very special cause of the error of those who first attempted to 



