No. 3-] LIMULUS POLYPHEMUS. 483 



evaginations of the walls of the sac, and that the outgrowths 

 were subsequently increased in length by the addition of ne- 

 phridial cells to their distal ends. These cells in turn become 

 hollowed out and united with the cells forming the nephric lobes. 

 In this way the system of tubules in the nephric lobes becomes 

 continuous with those leading into the end sac. 



There seem to be two sets of tubules opening into the sac ; 

 one set arises from its anterior wall and leads into the longitu- 

 dinal tubules of the stolon, and hence to the three anterior 

 nephric lobes ; the others lead into the tubules of the fourth 

 lobe. On its median side the sac opens through a small neck 

 into the nephric duct. 



Finally, in the adult, the end sac becomes so irregular through 

 the formation of the numerous large tubes opening into it that 

 its original boundaries cannot be distinguished. 



C. The nephric duct of the adult lies along the edge of the 

 plastron dorsal to the nephric lobes, and extends backwards 

 from the base of the second leg to the anterior side of the 

 sixth. The thin-walled transparent tube is easily torn, and, 

 unless injected, it is very difficult to trace out its various con- 

 volutions. On careful dissection the course of the duct is seen 

 to be as follows : At the distal end of the duct, just before it 

 opens to the exterior, is the ectodermic portion (PI. XXVIII, 

 Fig. 83, ect. and cut 3 B.). It is sharply marked off from the 

 rest of the duct by its thick walls lined with chitin. From 

 this point the duct turns at right angles and extends in a 

 dorsal direction, till it reaches the plastron, along the lateral 

 edge of which it extends as far as the first nephric lobe (PI. 

 XXVIII, Fig. 83). It then bends directly backwards, dimin- 

 ishing rapidly in size up to the angle of the second loop, which 

 in PI. XXVIII, Fig. 83, is seen on the median side of the 

 fourth nephric lobe. From here on the calibre of the tube 

 remains about the same. It now turns forwards, parallel to 

 the dorsal limb of the loop, as far as the posterior margin of 

 the third lobe, and then backwards to form a large mass of 

 coils, lying a little behind and dorsal to the fourth lobe. 

 From this coil the proximal end of the duct issues and passes 

 forwards and ventrally to the end sac, buried in the interior of 



