308 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



i ior end of the organ backwards. As a matter of fact this sequence 



n-ularity is interfered with by a tendency of the tubes 



p in batches. First a few tubules at the front end of the series 



;..|) one after the other constituting what is termed the pronephros 



. 128, B, pn). In those vertebrates which have a larval stage the 



tubules o! the pronephros become greatly enlarged, so that it is able to 



i lie whole excretory needs of the individual during this stage. 



tubules which would develop in the next succeeding segments do 



:nake their appearance being unnecessary owing to the great 



lopment of the tubules in front of them. It is only when it comes 



turn of a segment further back in the series to develop a tubule 



FIG. 127. 



i illii-tritiiin the arrangement of the nephridial tubes of Annelids (A) and Vertebrates (B). 

 ' I opening ; a.n.d, archinephric duct ; c, coelome ; ent, enteron ; n, nephridium. 



that the tubule actually makes its appearance. Successive tubules now 

 go on developing right back to the hinder end of the coelome, and the 

 whole df these constitute the opisthonephros (Fig. 128, B ; op}. The 

 opisthonephros takes on the excretory function hitherto performed by 

 the pronephros, and the latter, no longer required, rapidly dwindles 

 The opisthonephros forms the functional excretory organ or 

 kidney of the fishes and other lower vertebrates. 



In the higher vertebrates, from the Reptiles onwards, the opistho- 

 iH-plim-, becomes again differentiated into two portions an anterior 

 t he mesonephros which loses its excretory function in the adult and be- 

 related to the reproductive process and a hinder portion the 

 raetanephros (Fig. 128, 1) ; inn) composed of the hinder tubule or tubules 

 which be. nine immensely enlarged and added to by the development of 



