448 Causes and Course of Organic Evolution 



region that now could freely conduct such, not only were the 

 furrows converted into a first pair of pouches that came to 

 open internally; gradually additional posterior pairs devel- 

 oped till possibly 15-20 pairs may have existed. Of this vari- 

 able number Bdellostoma of the Cyclostomata may still have 

 from 14 to 11 pairs in one species (B. Stouii), 10 in another 

 (B. Dombeyi), or 7 to 6 in the Cape species which thus closely 

 resembles Petromyzon and Myxine. Amphioxus seems also 

 to have had from 11 to 9 primary pairs. In the branchiate 

 Batrachia, the sacs gradually become absorbed, so that from 

 5 to ultimately none are left, unless we continue to consider 

 the spiracular or hyomandibular area as the last modified 

 renuiant. 



It would be impossible here and now to enter into many 

 other striking morphological details, as for example the dis- 

 appearance of cilia from the outer faces of the gill pouch walls 

 or bars, in Amphioxus, and even from the whole body of the 

 adult, due e^ddently to the long-continued burrowing habits 

 of the animal. 



The excretory system in Rotifera and Rhabdocoela consists 

 of a pair of coiled tubes which run forward on either side of 

 the stomach and which each sends a coiled process forward 

 toward the neck. Each tube opens externally by a minute 

 ventral pore, and then branches copiously in the mesoderm 

 substance. The branches all end internally in "flame cells." 

 But in the higher or "alloiocoel" Rhabdocoelida — a few of 

 which such as Bothrioplana are fresh- water — the strong ceph- 

 alic and abdominal loops and transverse branches formed 

 between the main longitudinal canals (as seen in Fig. 5 p. 20 

 of Lank. Zool. IV) might be the commencing basis for a blood- 

 vascular system, as already suggested (p. 442). The finer 

 branches then of the excretory system would become more 

 and more separated off from the evolving vascular system, 

 though retaining intimate relations with it, alike in order to 

 draw off excretory products from the vascular fluid and to be 

 itself aerated. Such fine coiled branches then, ending in flame 

 cells, would constitute a primitive pronephros, as outlined 

 for nemerteans below. 



The excretory system in nemerteans shows a gradually evolv- 

 ing complexity from a simple pair of anteriorly placed tubes, 



