No. I.] STUDIES ON LIMULUS. 171 



{enta.'^'^). They then turn toward the median line in the epi- 

 dermis haemal to the heart, where they dip downward and com- 

 municate with the median nerve {m.c.n.) of the heart (PI. IX, 

 Figs. 5 and 6, and Text-fig. 8) opposite the last five pairs of 

 ostia {os.'^^^). The connections with the median nerve of the 

 heart have been actually found only for the cardiac nerves 

 {s.c.nP~'^) of the five branchial neuromeres, but similar connec- 

 tions probably exist in other neuromeres. 



Besides the branch communicating with the median cardiac 

 nerve, the segmental cardiac gives off numerous branches to 

 the epidermis haemal to the heart, and also an important branch 

 which goes anteriorly and unites with the pericardial nerve {p.n.), 

 a longitudinal nerve trunk running parallel to the heart inside 

 the pericardium. 



The cardiac branches {s.c.n^^'^^^) of the seventh and eighth 

 neuromeres fuse together and form a large nerve, which passes 

 haemally outside of the branchio-thoracic muscles and anterior 

 to the large entapophysis [enta.^^^^^). The root from the eighth 

 neuromere gives a branch to the lateral sympathetic. Upon 

 the haemal side of the body this cardiac nerve divides into a 

 number of branches. A large one enters the inter-tergal mus- 

 cle and breaks up into anastomosing branches which supply 

 that muscle. Some small branches pass into the epidermis 

 haemal to this muscle, and approach the median line. Although 

 these branches could be traced in the epidermis to points just 

 above the three anterior pairs of ostia {os.^^), no connections 

 with the median nerve of the heart could be made out. In this 

 region the connective-tissue strands which support the heart 

 upon the haemal side are very numerous, and it is difficult to 

 trace nerve fibers among them. 



The most important branch of this cardiac nerve, the peri- 

 cardial nerve {p.n.), turns posteriorly in the areolar tissue which 

 lies above the pericardial sinus. It first gives off a small 

 branch to the lateral inter-tergal muscle, and then, continuing 

 posteriorly, gives a branch to each of the cardiac nerves 

 (s.c.n.^^^) of the branchial neuromeres. These branches pass 

 from the outer side of the pericardial nerve toward the proxi- 

 mal ends of the cardiac nerves. The posterior extremity of the 



