EXCRETORY SYSTEM. 397 



purpose. In the embryo the blood is kept normal, as will 

 be explained afterwards, by aid of the birthrobe known as 

 the allantois ; and after birth the animals breathe by lungs. 

 All adult Amphibians also have lungs, to which the air- 

 bladder of Dipnoi is physiologically equivalent. 



The gill-clefts arise as outgrowths of the endodermic gut 

 which meet the ectoderm and open. The ventral paired 

 lungs arise from an outgrowth of the gut, and such also is 

 the air-bladder of most Fishes, though that usually lies on the 

 dorsal surface, has rarely more than a hydrostatic function, 

 and has a blood-supply different from that of the lungs. 

 That lung and air-bladder are homologous is by no means 

 certain, but the comparison is plausible. 



Exa-etory System. — From Cyclostomata onwards there is 

 a paired kidney. In the embryo this consists of a number 

 of segmentally arranged tubules or nephridia, and these are 

 connected on each side by a longitudinal segmental duct, 

 which seems to have an epiblastic origin, and extends from 

 the front of the body-cavity to the cloaca. The primitive 

 tubules are comparable to, and perhaps homologous with, 

 the nephridia of Annelids. According to their position they 

 are distinguished as forming head-, mid-, and hind-kidney, or 

 pro-, meso-, meta-nephros ; but these three regions are never 

 developed in one organism ; and some of the original tubules 

 usually lose their excretory function or atrophy. In its 

 typical form, each nephridium consists {a) of an internal 

 ciliated funnel (nephrostoma) opening into the body-cavity, 

 but rarely persistent ; {b) of a dilatation (Malpighian body) 

 into which blood-vessels project ; and {c) of a coiled tube in 

 part excretory, in part a conducting canal for the waste 

 filtered from the blood. The segmental or archinephric 

 duct on each side is usually divided into, or it may be 

 replaced by two ducts, the Miillerian duct and the mesone- 

 phric or Wolfifian duct. The Miillerian duct becomes the 

 genital duct or oviduct of the female, while in the male the 

 Wolfifian duct becomes the genital duct or vas deferens. The 

 ureters or ducts from the persistent functional kidneys, are 

 either the archinephric ducts {e.g., in Cyclostomata), or the 

 Wolffian ducts (in Amphibians), or special posterior deriva- 

 tives of the latter. 



