THE UROGENITAL SYSTEM 



293 



the course of j^hylogenesis, they have changed from a reproductive to an 

 excretory function. In this connexion it is interesting to note that the 

 coelomoducts of molluscs are excretory in function, and that in male 

 vertebrates some of the anterior tubules of the mesonephros serve as 

 reproductive outlets. 



Acrania. Among the protochordates, the urochordates appear to 

 have no organs comparable with either nephridia or coelomoducts. The 

 hemichordates, with the possible exception of Rhabdopleura, have no 

 paired excretory organs. The proboscis cavity of Rhabdopleura contains 

 a couple of ciliated tubules which are usually denominated nephridia. 

 We find, however, in Amphioxus, protonephridia strikingly similar to 



,„ DORSAL 

 -ia' AORTA 



'S-PROTONEPHRIDIUM 



VESSEL 

 SOLENOCYTES, 



CAPILLARY. 



'NEPHROPORE 



Fig. 263. — Diagrams of the protonephridia of Amphioxus. A is a section in the 

 pliaryngeal region — through a gill septum on the left and through a gill slit on the right. 

 B is an enlarged section of a protonephridium showing the attached solenocytes. Such 

 a protonephridium is practically identical in structure with a protonephridium of 

 annelids. (Redrawn after Boveri and Goodrich.) 



those of annelids. Their presence in chordates is frequently interpreted 

 as supporting the assumption that chordates have an annelid ancestry. It 

 seems more likely, however, that we have here another case of convergence. 



The nephridia of Amphioxus are protonephridia associated with the 

 secondary gill arches. They are short branched tubules which open into 

 the peribranchial cavity, sometimes as many as ninety pairs. That the 

 nephridia of Amphioxus are protonephridia comparable with those of 

 anneHds is evidenced by the fact that they are ectodermal and meta- 

 merically arranged and have their blind terminations beset with soleno- 

 cytes almost identical in structure with those of annelids. (Fig. 261) 



Craniota. Three successive kidneys make their appearance in verte- 

 brates, pronephroi, mesonephroi, and metanephroi. That the tubules 

 of these three kidneys are comparable with coelomoducts is sufficiently 

 attested by their mesodermal origin and, with the exception of the tubules 

 of the metanephros, their metameric arrangement. The pronephros in 



