No. I.] STUDIES ON LIMULUS. 149 



They arise upon the haemal side of the neural nerve, the two 

 former very near the brain, the latter or median one a little farther 

 out. Sometimes there are two additional nerves supplying the 

 inner plastro-coxal muscles, but these may be regarded as 

 branches of the main ento-coxal nerves, for in many specimens 

 these muscles are supplied by nerves which are undoubted 

 branches of the main ento-coxal nerves. 



The anterior ento-coxal nerve {a.e.n.) arises from the anterior 

 haemal side of the neural nerve quite close to the brain, gives off 

 a small branch to the innermost, anterior, plastro-coxal muscle 

 (J'^), then passes down through the substance of the nephrid- 

 ium (PI. VII, Fig. 2, it.^ ) and innervates all the muscles (J"^''), 

 which are inserted upon the anterior border of the entocoxite, 

 but apparently gives no branches to the nephridium. It termi- 

 nates in delicate filaments in the areolar tissue at the outer 

 extremity of the entocoxite, and sends some fibers to the ante- 

 rior sensory knob. 



The posterior ento-coxal nerve [p.e.n.) arises from the poste- 

 rior, haemal side of the neural nerve, and innervates all the 

 muscles which are inserted upon the posterior border of the 

 entocoxite. It also passes through the substance of the ne- 

 phridium, but does not give off any nerves to it. It terminates 

 in areolar tissue at the outer extremity of the entocoxite and 

 innervates the posterior sensory knob. 



The median ento-coxal nerve (in.e.n.) arises from the haemal 

 side of the neural nerves some distance farther from the brain 

 than the other ento-coxal nerves. It is much smaller than these 

 nerves, and passes out over the surface of the nephridium accom- 

 panied by a blood vessel. It is entirely sensory in function and ter- 

 minates in the median sensory knob and the surrounding tissues. 



In the second appendage (PI. VII, Fig. 2) the anterior and 

 posterior ento-coxal nerves [a.e.n.^ and p.e.n.-) are similar to 

 those in the third appendage, except that separate nerves supply- 

 ing the inner plastro-coxal muscles are often present. The 

 median nerve has not yet been found, but it probably exists, and 

 has been overlooked on account of its extreme tenuity. All the 

 median ento-coxal nerves, except the sixth, are very small and 

 difficult to find. 



