294 CHORD ATE ANATOMY 



vertebrates is an embryonic structure which persists only in the adults of 

 some cyclostomes, teleosts, and dipnoans. The mesonephros becomes the 

 functional kidney of adult Anamnia, the metanephros of adult Amniota. 



Comparative embryology and anatomy suggest, therefore, that the 

 function of excretion has phylogenetically migrated from the anterior 

 to the posterior part of the body. The possibility, however, cannot be 

 excluded that originally every metamere in chordates, as in annehds, 

 contained a pair of excretory organs. 



Pronephros. The first vertebrate kidney to develop in ontogenesis, 

 and possibly also the oldest in phylogenesis, is the pronephros or head 

 kidney. This consists of three to fifteen pairs of segmental tubules, each 

 of which opens by a nephrostome into the body-cavity. Each tubule is 

 connected laterally with the primitive pronephric duct which carries excre- 

 tions posteriorly to the cloaca. It is possible that originally each pro- 



PROTONEPHRIDIUM PRONEPHROS MESONEPHROS METANEPHROS. 



PROTONEPHRIOIA RtMAL TUBUL£S CLOMERUi.1 WOLf FIAN DUCT 



SLCfiETCPlY TUBULXS. 



COLLECTING TUBULES'. 



CHORDATE KIDNEYS. u(I£TEr' 



Fig. 264. — A diagram illustrating the four types of kidneys which occur in chordates. 

 The excretory tubules of amphioxus are ectodermal in origin like the protonephridia of 

 annelids, while they are mesodermal in vertebrates. In the course of phylogenesis the 

 excretory organs of chordates have migrated farther and farther back in the body, as is 

 shown in the diagram. 



nephric tubule, like the coelomoducts of anneUds, had its own aperture. 

 The elimination of liquid wastes by a common pronephric duct must 

 be considered as a secondary condition. The factors in this phylogenetic 

 development are obscure. 



The pronephros functions in few adult vertebrates, but it appears to 

 function in the early ontogenesis of those craniotes which have httle yolk 

 in their eggs and which consequently have a prolonged larval period. 

 Usually pronephric tubules are not connected with glomeruli. The 

 associated glomeruli, instead of connecting directly with the tubules 

 through a Bowman's capsule, project into the body-cavity in the neigh- 

 borhood of the nephrostomes. In this way wastes excreted into the body- 

 cavity find their way indirectly to the tubules of the pronephros and 

 thus to the cloaca. In some cyclostomes and bony fishes, however, the 

 pronephros includes a segregated portion of the body-cavity. This 

 becomes a pronephric chamber into which an inner glomerulus projects. 



Mesonephros. The mesonephros is the functional kidney of Anamnia. 

 Its tubules, like those of the pronephros, are mesodermal and derived 

 from the intermediate cell mass, the nephrotome. They utilize the primi- 

 tive (pronephric) duct as an outlet. From their structure, development, 



