iv VERMES NEPHRIDIA 243 



The nephridia as organs of excretion. The original and most 

 sceneral function of the nephridia is that of excretion. The discharge 



O O 



of the excretory products occurs in two ways. The nephridia may 

 either take up the excretory products direct out of the body fluid 

 or out of the blood by means of the open funnels, or the excretory 

 products may collect in the walls of the nephridia and thence reach the 

 nephridial cavity. The nephridia are often surrounded by a rich net- 

 work of blood-vessels which yield up to the nephridial Avails the 

 excretory material they contain. 



The nephridia as duets for the sexual products. Since the 

 nephridia establish an open means of communication between the body 

 cavity and the outer world, an opportunity is offered to the sexual pro- 

 ducts floating in the ccelomic fluid to choose this way of reaching the 

 exterior. In the Polyckceta the nephridia in fact act at the same time 

 as sperm ducts and oviducts. In the simplest cases this new function 

 does not bring about any marked variation in the form and struct- 

 ure of the nephridia, the funnels at most showing slight enlargement 

 at the time of sexual maturity. It often happens, however, that some 

 of the nephridia act almost exclusively as sperm ducts and oviducts. 

 Their funnels are then strikingly enlarged, while at the same time the 

 (excretory) nephridial canal diminishes in size and becomes simplified. 

 Some of the nephridia in the Capitellidce undergo even more profound 

 modification. The funnel becomes enormously enlarged, and connected 

 with the exterior by means of a new canal, which breaks through the 

 body wall and opens outwardly through a genital pore, while at the same 

 time the nephridial canal may become reduced and may cjuite disappear. 

 Thus arise the genital tubes ; they act as ducts for the sexual products 

 and as copulatory organs. The sperm ducts and oviducts of the Oliyo- 

 chceta have also long been considered as modified nephridia, and it can- 

 not be denied that they show great agreement with nephridia in their 

 structure and composition (funnel, canal, or duct and terminal vesicle) 

 as well as in their development. Nevertheless it is a striking fact 

 that in the Oligochceta, where the nephridia occur in strictly segmental 

 arrangement (one pair in each segment), either permanent (Lumlricida'} 

 or provisional nephridia (other Oligochceta} are found in the genital 

 segments also, side by side with the sperm ducts and oviducts. In 

 Acanthodrilus, where 4 pairs of nephridia occur typically in each 

 segment, there are also 4 pairs in the genital segments. These are 

 difficulties which cannot be ignored in the way of establishing a 

 homology between the oviducts and sperm ducts of the Oligochceta and 

 nephridia. The ducts for the sexual products in the Hirudinca and 

 Myzostomidce can certainly not as yet be considered as modified 

 nephridia. 



The phylogenetic origin of the nephridial system of the Annulata is still 

 quite uncertain. There are three different views. According to one of these, the 

 whole nephridial system of the Annulata corresponds with the water -vascular 

 system of the Platodcs and with the excretory system of the Noncrtiiw, which (in the 



