4o6 



THE EVOLUTION OF MAN. 



toma. Although both orders of this class — the Myxinoides 

 as well as the Petromyzontes — possess developed, urine- 

 secreting kidneys, these organs do not in this case serve to 

 carry away the sexual cells. These cells pass directly from 

 the germ-glands into the coelom, and are discharged through 

 a posterior aperture in the abdomen. The condition of the 

 primitive kidneys in these is, however, very interesting, and 

 A throws light on the complex kidney 



structure of the higher Vertebrates. 

 In the first place, in the Myxi- 

 noides {Bdellostoma) we find a long 

 tube, the primitive kidney duct 

 {prohireter, Fig. 317, a), on each 

 side. This opens internally into 

 the coelom through a ciliated funnel- 

 shaped aperture (as in Ringed 

 Worms) ; it opens externally through 

 an opening in the outer skin. A 

 great number of small horizontal 

 tubes (" segmental canals," or primi- 



FiG. 317.— .4. Portion of kidney of Bdel- 

 lostoma : a, primitive kidney duot (jirotu- 

 reter); h, segmental canals, or primitive 

 urine canals {tuhuU uriniferi) ; c, kidney- 

 vesicles {capsules Malphigianw^. — B. Por- 

 tion of the same, much enlarged : c, kidney- 

 vesicle, with the ijlomerulus ; d, approaching 

 artery ; e, retreating artery. (After Johannes 

 Miiller.) 



tive urine tubes) open on its inner side. Each of these 

 terminates in a blind, vesicular capsule (c) enclosing a 



