4 o6 



THE EVOLUTION OF MAN. 



toma. Although both orders of this class the Myxinoidea 

 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 ccelom, and are discharged through 

 a posterior aperture in the abdomen. The condition of the 

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

 throws light on the complex kidney 

 structure of the higher Vertebrates. 

 In the first place, in the Myxi- 

 noides (JJdellostoma) we find a long 

 tube, the primitive kidney duct 

 (protureter, Fig. 317, a), on each 

 side. This opens internally into 

 the cffilom through a ciliated funnel- 

 shaped aperture (as in Kinged 

 Worms) ; it opens externally through 

 an opening in the outer skin. A 

 great number of small horizontal 

 tubes (" segmental canals," or primi- 



FIG. 317. A. Portion of kidney of Bdel- 

 lostoma : a, primitive kidney duct (protu- 

 reter); b, segmental canals, or primitive 

 nrine canals (tubuli uriniferi) ;, kidney- 

 vesicles (capsulae Malphigiana). B. For- 

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

 vesicle, with the ylomerulus ; d, approaching 

 artery; e, retreating artery. (After Johannes 

 Muller.) 



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

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



