608 THE AMERICAN NATURALIST.  [Vor. XXXVI. 
running the entire length of the half body to which it belongs, 
meet in the middle line to form one tube. This mode of origin 
is in harmony with that seen in the inner iliac and ischial bones 
and the median hind leg. 
The vascular system shows a similar arrangement. Anteriorly 
there are two complete systems, while posteriorly the systems 
are complete on the outside, but not entirely so in the central 


genital org 1 ls: a.4., anterior bladder ; 4.4/., posterior 
bladder; æ., urachus; ur., ureter; s.e., umbilical artery; Z.2., iliac artery; Z.v., iliac 
^, Vas ens, 
vein ; £., testis ; v. d. defer 
area. There are two dorsal aortze (Fig. 7), and the renal and 
spermatic arteries are in pairs on both sides, as if the bodies 
were independent. Posteriorly, however, the aortz each give 
rise to one large iliac artery to go to the paired leg of that side, 
and a second which was not traced beyond the root, where it 
soon disappeared among the muscles. At the fork of these 
' iliacs a single umbilical artery arises from each aorta; it runs 
