90 CLASS PISCES. 



deferens, but disappears or is reduced to a slight vestige in the 

 female. 



In Elasmobranchs the longitudinal duct is at first called the 

 segmental duct on the view that the Miillerian duct is segmented 

 off it. As we have seen, this is not a good description of what 

 happens, and the name is not a happy one. After the Miillerian 

 duct has become distinct from it, it becomes the duct of the per- 

 sisting kidney, and eventually, owing to the shifting back of the 

 point of opening of the metanephric tubules, the mesonephric 

 duct. 



The nephridia typically open directly by the so-called collect- 

 ing tubules into the part of the longitudinal duct opposite to 

 them, but with the differentiation of the metanephros the collect- 

 ing tubes of the posterior nephridia shift their point of opening 

 into the longitudinal duct backwards, so that they all come to 

 open close together into the longitudinal duct — now called 

 mesonephric duct — close to the cloaca. They are usually re- 

 ferred to as ureters. 



The development of the nephridia of the part of the kidney 

 behind the pronephros, as direct transformations of a portion 

 of the coelom occurs only in Elasmobranchs. In other fishes 

 the development of these tubules is delayed until the myotomes 

 and adjacent tissues have become functional, and have lost their 

 primitive relations. The consequence is that the development is 

 modified and the nephridia (except of the pronephros) are de- 

 veloped from small nodules of growing tissue, which make 

 their appearance during larval life in the proper positions. 



Abdominal pores, as distinct from generative pores, are pre- 

 sent in most Elasmobranchs, some Teleostei and in Ganoids, 

 but they are strangely variable in their occurrence. They never 

 act as generative outlets, and their function would appear to be 

 for the outlet of excretory substances of the body-cavity itself. 

 AsBles has pointed out, they are rarely present in forms in which 

 the nephrostomes of the kidneys are persistent. 



Generative Organs. — Excepting in certain forms, such as 

 Serranus and Chrysophrys, which are hermaphrodite, fishes 

 are of separate sexes ; the two sexes sometimes present 

 external differences. The male and female reproductive 

 organs often resemble one another so closely in form and position 

 that it is necessary to investigate their contents in order to dis- 



