STUDIES ON LIMULUS. 197 



nerves (j.c. «.'""). The pericardial nerve also gives a branch to the inter-tergal 

 muscle (i.Tn.) which is attached to the first entapophysis. These two anterior 

 nerves (s.c.n.^ and s.c.mJ^^'^^) have not been observed to connect with the cardiac 

 plexus. 



The cardiac nerves {s.c.n.'^''^^) from the five branchial neuromeres send numer- 

 ous branches to the epidermis, and also branches to the median cardiac nerve {m.c.n.) 

 opposite each of the last five pairs of ostia [os.'^'^^). There is considerable varia- 

 tion in the arrangement of the nerves in the posterior portion of the haemal 

 region. Moreover, the branches are very numerous and very fine, so that many of 

 them are necessarily missed in dissection. Some of the branches are continued 

 posteriorly to the margin of the carapace. 



The two post-cardiac nerves (s.c.n.^*'^"'^'^) send branches in the epidermis to 

 the haemal line. Branches from the last cardiac {s.c.n.^^) and from the two post- 

 cardiac nerves (j-.it.w.'-* ^""^ ^s) anastomose, and innervate one of the slips of the 

 extensor (t.e.mA) of the telson. Other branches of the last post-cardiac nerve 

 innervate the other extensor (t.e.mX>) and the epidermis of the posterior margin of 

 the abdomen. 



Fig. 6. A vertical section through the abdominal region in the ninth and tenth 

 neuromeres showing the course of one of the cardiac nerves (from the anterior 

 side, about twice natural size). 



The left half of the drawing represents the posterior portion of the ninth neu- 

 romere, and the right half the anterior portion of the tenth neuromere. 



The heart {ht.) is seen in cross-section showing a central lumen surrounded by 

 a layer of anastomosing muscle fibers ; the median and lateral cardiac nerves 

 {vi.c.ti. and l.c.n.) are shown in the three angles of the heart. 



Upon the right, one of the alary muscles {al.m}°) runs from the lateral angle 

 of the heart to the pericardium. 



The pericardial sinus {p-s.) surrounds the heart and the branchio-cardiac canal 

 {b.c.c}°) is seen coming from the upper portion of the second gill {ap}°). 



A pericardial membrane (/.) bounds the pericardial sinus upon the neural side. 

 It is attached to the entapophysis {enta.'^) upon the left and to the branchial carti- 

 lage {b.c}°) upon the right. The veno-pericardiac muscles (z/./.w.'") are attached 

 to it, and the collateral arteries {car.) are found in close connection with it. 



The intestine (int.) lies haemal to the ventral cord which is enclosed in the 

 ventral artery (v.ar.). 



Neural to the ventral cord is the abdominal endochondrite {a.e.'^) which fur- 

 nishes attachment for the haemo-neural muscles {h.ti.ni.'^) and the internal branchial 

 muscles {i.b.m.'^). The external branchial muscles (e.b.m.^ ^'^^ '°) are also seen, 

 and the longitudinal abdominal muscles {l.a.tn.) and branchio-thoracic muscles 

 [b.t.m.) are cut across. The veno-pericardiac muscle (v.p.m.^°) is seen attached to 

 the pericardium, and spanning the venous-collecting sinus (z'.c.s.) at its neural end. 



An abdominal ganglion with its neural (n.n.^) and haemal nerves (h.71!^) is 

 represented. The neural nerve and the integumentary portion {in.n.'^) of the 

 haemal nerve are cut off. 



The intestinal nerves {i.ti.^) arise from the haemal nerve close to the ganglion; 

 the anterior branch {a.i.n.) goes to the longitudinal abdominal muscles {l.a.m.), 

 and the posterior one {p.i.n.) goes to the haemo-neural muscle (h.it.m.^) and to 

 the intestine. 



The cardiac branch {s.c.n."^) arises opposite the branchio-thoracic muscle, and 



