UROGENITAL SYSTEM OF MYXINOIDS 109 



appearance of having at one time extended farther forward. 

 There is no uniformity in the diameter of the duct at its anterior 

 end, except that it is always smallest here, varying in width from 

 a mere thread to 1 mm. 



The left duct usually bends abruptly laterad at its anterior 

 end, until it assumes a position close beside the left postcardinal 

 vein, and retains this position to the end of the body cavity. The 

 right ducts bends only sUghtly laterad at its anterior end to 

 assume a similar position beside the right postcardinal vein. 

 The right duct is invariably closer to the dorsal aorta than the 

 left. 



In adults of the same size, the ducts of the males are larger 

 and longer than those of the females. There is not much varia- 

 tion in the size of the two ducts of a given animal, nor in the ducts 

 of all the adult animals of the same sex. Each duct is smallest 

 at its anterior end, and gradually increases to its largest size at 

 the posterior end of the body cavity, where it is from 1.5 to 2 mm. 

 wdde in females and from 3.5 to 4 mm. in males. 



Each duct is flattened dorso-ventrally throughout its entire 

 length, being most flattened at the posterior end of the body 

 cavity. The outer surface of the ducts is smooth. When a 

 duct is stained, cleared and examined under the microscope the 

 surface appears longitudinally striated, due to the occurrence of 

 ridges formed in the lumen of the duct by the columnar epithelial 

 Uning. 



The ducts lie immediately ventral to the notochord, separated 

 from the latter by a small cavity which extends the entire length 

 of the ducts. By cutting the peritoneum along the dorsal body 

 wall at the sides of each duct, the ducts, together with the post- 

 cardinals and the dorsal aorta, can easily be removed. 



The mesonephric ducts of the adult male are unlike those of 

 the adult female in some respects. Those of the former are not 

 only larger, but are longer, by reason of lateral bendings or con- 

 volutions. Figure 45, a dorsal view, represents the typical ap- 

 pearance of the male ducts, and figure 46 those of the female. 

 At their anterior ends the ducts of the male are only slightly con- 

 voluted, but, beginning about ten somites posterior to the ante- 



