486 PATTEN AND HAZEN. 



the Trilobite stage (PI. XXV, Fig. 43, 7t.c^). These cells first 

 become granular, then vacuolated, and then united end to end 

 (PL XXVIII, Fig. 79). In Limuli about three-quarters of an 

 inch long the cells were still present, but they were not united 

 with the permanent nephric lobes, and appeared to be degen- 

 erating. 



X. Summary. 



1 . Branchial Cartilages. — A thick ring of somatic mesoderm 

 forms at the base of each abdominal appendage. The gill 

 cartilage arises as a plate of somatic mesoderm attached by its 

 dorsal end to the ventral wall of the somite, and continuous on 

 either side with the ring of mesoderm. The ventral end of 

 the cartilage finally extends through and beyond the mesoder- 

 mic ring and becomes attached to the anterior wall of the 

 corresponding appendage. 



2. The ventral ends of the abdominal somites persist as venus 

 sinuses. 



3. The genital ducts arise as diverticula of the median 

 ventral side of the opercular somite. They remain in a rudi- 

 mentary condition until after the second larval stage. 



4. Nephric Duct. — A nephric plate is formed from a single 

 layer of columnar cells of the somatic mesoderm on the median 

 side of the fifth somite. The plate is gradually evaginated to 

 form a funnel, opening by a wide mouth into a thin-walled end 

 sac that represents the fifth somite; the opposite end unites 

 with a shallow ectodermic invagination at the base of the fifth 

 leg. The tube becomes much convoluted, and is converted 

 directly into the adult nephric duct. Finger-like outgrowths 

 of the end sac finally unite with the hollow cell chains of the 

 adjacent nephric lobes. 



5. The Nephric Lobes. — A mass of nephric cells arises inde- 

 pendently of the duct from the median dorsal portion of the 

 somatic layer of each of the six thoracic somites. The cells 

 become enlarged and filled with coarse granules. The granules 

 become smaller or disappear, and a vacuole appears in each 

 cell. The latter elongate and unite end to end to form irregu- 

 lar masses of branching intracellular tubules. The cell masses 



