651 



likely, and fortunately it is not the only explanation that can be 

 given. 



In a recent paper, in Nature^) J. T. Cunningham says "In La- 

 nice con chile ga the uephridea have coalesced together after 

 coming in contact from before backwards, the separating membranes 

 havnig disappeared. The case is extremely interesting in 

 the fact that we have in it an approximation to the 

 condition of the excretory system in Vertebrata'-^): the 

 presence of a metameric series of nephrostomata in Vertebrate embryos 

 has long been seen to constitute a resemblance between them and 

 the Chaetopoda, but hitherto no Chaetopod was known which resembled 

 the Vertebrate in having a number of nephridia coalesced to form a 

 continuous longitudinal tube". 



It is impossible to gather from this statement to what extent 

 Cunningham believes these coalesced nephridial ducts to be homo- 

 logous with the segmental duct of Vertebrates. He obviously ignores 

 entirely the epiblastic origin of the latter, and I for one cannot re- 

 gard the comparison as a serious one. 



This comparison also must be classed in morphological worth 

 with that of Ransome and Thompson, and that of Hasse. 



The two things are toto coelo difi'erent in origin, and morpho- 

 logically admit of no comparison. But none the less, we 

 have now, owing to the certainty of the epiblastic origin of the 

 pronephric duct, gained an important position for a more certain 

 comparison of Vertebrate and Annelid nephridia. The phylogeny 

 of the system is indeed much clearer, for, obviously we 

 are entitled to assume that the Annelid ancestors of 

 Verteb rates possessed a series of segmental nephridia, 

 which opened into a longitudinal groove on each side 

 of the body, that for some reason, possibly owing to 

 an increase in size of the cloaca, and possibly becaus e, 

 apparently, every groove tends to become a tube^), the 

 groove which extended as for as the nephridia, i.e. to 

 the cloaca, got folded in to form a tube, and so came to 

 open into the cloaca. 



1) Nature, June 16»' 1887, p. 162. The Nephridia of "Lanice 

 coiichileo;a" Malmgren. 



2) The Italics are miue. 



3) The term pronephric duct is, it appears to me, morphologically 

 unsound, segmental duct is better. 



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