No. 3.] 



LIMULUS POLYPHEMUS. 



481 



A proliferation of the ectoderm on the posterior median side 

 of the fifth leg is seen in PI. XXVII, Fig. 63, ect.p.., mark- 

 ing the beginning of the ectodermic infolding, which in the 

 following stage unites with the distal end of the nephric 

 duct. 



In the next stage (PI. XXVII, Figs. 65-70) the distal end of 

 the duct has united with the ectodermic infolding (PI. XXVII, 

 Fig. 71). The nephric plate is still visible as the flaring mouth 

 of the duct (PI. XXVII, Figs. 65 and 66), the lateral lip being 

 much longer than the median one. 



Ttia- 



FiG. 3 A. — Diagram of the 2d larval stage, showing nephridial cells in the chelicera, the 2d, 3d, 

 4th, 5th, and 6th legs. The end sac is at the base of the 5th leg, surrounded by nephridial 

 cells. The duct extends forward to the 2d leg, and backward to the 6th. The proximal 

 arm is much coiled, the distal arm is nearly straight. 



Fig. 3 B. — Diagram representing the adult condition. The median ends of the lobes have 

 grown forward and backward and united to form the stolon. This portion of the nephridia 

 is composed of large collecting tubules, which carry the excreta to the end sac in the 4th 

 lobe; from there they pass into the nephridial duct. 



The relative size and positions of these parts are shown in a 

 little earlier stage in PI. XXVI, Fig. 58. 



In the next stage the duct elongates very rapidly, and as each 

 end is fixed a fl -shaped tube is formed with the loop reaching 

 forward to the middle of the fourth leg. 



The ectodermic portion of the duct is very short (PI. XXVII, 

 Fig- 73)» ^n<i nia-y be readily distinguished by its small deeply 

 stained nuclei and by its delicate internal lining of chitin. 



