No. I.] 



STUDIES ON LIMULUS. 



159 



l).t.-m. jrd 



Fig. 16. — Diagram showing the muscles and distribution of nerves in the first gill. The appendage 

 is flexed upon the abdomen, and is seen from the neural side (about i^ natural size). 



a.e.'i, abdominal endochondrite ; b.c.^ and 9, branchial cartilages of operculum and first gill ; 

 i.L, inner lobe of gill ; 771.I., median lobe of gill ; o.L, outer lobe of gill. 



Muscles: a.b.m.'i, abductor muscle of gill; b.t.m., branchio-thoracic muscles; e.b.m.'i, 

 external branchial muscle; i.b.in.'), internal branchial muscle; i./.in., inner lobe muscles; 

 l.a.m., longitudinal abdominal muscles ; o.l.tn., outer lobe muscles. 



Nerves: a.g-., first abdominal ganglion; e.h.n., external branch of neural nerve; g.n., 

 branch of neural nerve supplying gill book ; h.n^, haemal nerves ; i.b.n., internal branch of 

 neural nerve; z'.«.9, intestinal nerve (two branches are shown, a posterior and an anterior one); 

 in.n.9, integumentary branch of haemal nerve; i.s.n., lateral sympathetic nerve ; »«.^.«., median 

 branch of neural nerve; «.;/. 9, neural nerve ; s.c.a.^, segmental cardiac nerve ; z'.c, ventral cord. 



