Evolution of Fishes from Invertebrates 91 



from ten reducing to six pairs in number, formed the 

 mesonephros, that strongly suggests the middle set of ne- 

 mertean tubules. The two longitudinal ducts of these now 

 end behind in a common urinary sinus which opens into the 

 common cloacal area. But while the mesonephric tubules 

 were at first strictly symmetrical, "in myxinoids only do 

 they remain so. In other craniates, at all events through- 

 out the greater length of the mesonephros, a varying num- 

 ber of new tubules arise from masses of cells nipped off 

 from the first rudiment. All of the mesonephric tubules 

 are therefore derived from the same original series of 

 rudiments by a sort of budding. These secondary tubules 

 acquire the typical structure and relations," but since the 

 tubules then became crowded "their metameric order is 

 lost." (_5/:87). 



Until more minute comparative study has been made it 

 would be impossible to say whether the most posterior 

 tubules of some metanemerteans are the forerunners of the 

 supposed metanephros of male elasmobranchs, and the 

 metanephros of higher vertebrates. Such seems a likely 

 explanation. The abdominal pores of most fishes would 

 appear, as noted above, to be surviving remnants of one 

 pair of tubule-pores seen in metanemerteans. Their paired 

 persistence in the most ancient classes (Dipnoi, Crossop- 

 terygii, Holostei, Chondrostei, and most Elasmobranchll), 

 their occasional persistence only in a few Teleostei (Mor- 

 myrldae, Salmonldae), and their disappearance In most 

 Teleostei and In some specialized Elasmobranchll, favor 

 such a conclusion. 



The higher nemerteans are dioecious as a rule, though 

 a few species of Tetrastemma seem to be truly herma- 

 phrodite. The former condition Is practically constant for 

 vertebrates, but a tendency to primitive hermaphroditism 

 is shown by Myxine, where sperms may be produced In 

 a younger, and eggs In an older stage of the animal. 



XIII. The reproductive organs. 



In Nemerteans the reproductive sacs are arranged in 

 numerous metameric pairs along the sides of the animal, and 

 alternate with the often paired lobes of the digestive tract. 



