402 



COMPARATIVE ANATOMY 



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 acrania, protonephridia strikingly similar to those of 

 annelids. Their presence in chordates is frequently interpreted as sup- 



SOLENOCYTES 



...DORSAL 



f AORTA CAPILLARY, 



— PROTONEPHRIDIUM 



PHARYNX 



PERI BRANCHIAL 



'NEPHROPORE 



Fig. 331. — Diagrams of the protonephridia of Amphioxus. A is a section in the 

 pharyngeal 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.) 



porting 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. The fact 

 that they are ectodermal in origin and are limited to the gill-region, while 

 the excretory tubules of vertebrates are mesodermal and postbranchial, is 

 sufficient to show that the two are not homologous. That the nephridia of 

 amphioxus are protonephridia comparable with those of annelids is 

 evidenced by the fact that they are ectodermal and metamerically arranged 

 and have their blind terminations beset with solenocytes almost identical 

 in structure with those of annelids. 



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 



