iv NEPHBIDIAL OKGANS 221 



circulating mesenchyme or blood. This would render possible the 

 shortening up of the nephridial tubes and the more definite localiza- 

 tion of the excretory tissue. Whereas the original flame-cell type 

 of excretory apparatus was diffuse— the flame-cells being scattered 

 throughout the mesenchyme sponge-work — as it is still to be seen 

 in the more lowly organized forms — it would now become compact, 

 the waste-products being brought to it by the movements of the 

 circulating blood. 



Nephridial Organs of Vertebrates. — Before passing on to the 

 details of development of the renal organs in Vertebrates it is 

 necessary to notice one or two points of general importance regard- 

 ing the morphology of these organs within this particular phylum, 

 and also to define precisely the sense in which certain technical 

 terms will be used. 



In the first place the kidneys or renal organs of Vertebrates are 

 built of tubules each of which is a nephridium according to the 

 original definition of the term. 



The conclusion, already arrived at, that the ancestral Vertebrate 

 possessed a completely segmented coelome, carries with it the further 

 conclusion that in all probability a pair of nephridial tubes originally 

 opened to the exterior from each segment. A characteristic feature 

 however of the Vertebrates (with the exception of Amphioxus) is 

 that the nephridia open not directly to the exterior on the surface of 

 •each segment as in a typical Annelid but into a longitudinal duct 

 which passes back along each side of the body and communicates at 

 its hind end with the cloaca. The whole series of nephridial tubes 

 on each side of the body is known as the archinephros J and the 

 duct as the archinephric duct. 



In the embryos of Vertebrates development takes place from the 

 head end backwards. We should therefore expect the nephridial 

 tubules to appear in regular sequence from before backwards. It is 

 however highly characteristic of the Vertebrate that the tubules, 

 instead of developing in this regular sequence, develop in three 

 batches one behind the other — an anterior, a middle, and a posterior. 

 These constitute respectively the pronephros, mesonephros, and 

 metanephros (Lankester, 1877). In many of the lower Vertebrates 

 there is no separation between mesonephros and metanephros, the 

 two forming a continuous structure which acts as the functional 

 kidney. Such a type of renal organ consisting of the series of 

 tubules corresponding to mesonephros together with metanephros 

 may conveniently be termed the opisthonephros. 2 



Of the four types of kidney just mentioned the first — the pro- 

 nephros — forms the functional kidney in larval Vertebrates. It is 

 well seen in the larvae of Crossopterygians, Actinopterygians, Lung- 

 fishes, and Amphibians, while, as Sedgwick first pointed out, it is 



1 Archinephron, Lankester (1877). Price's term Holonephros is also frequently- 

 used in the same sense. 



s In analogy with the use of the word opisthosoma in the group Arachnida. 



