No. I.] STUDIES ON LIMULUS. 163 



Fig. i) proceed diagonally backward and outward to the rim of 

 the carapace. The fibers show a tendency to separate from 

 each other as they approach the thicker margin and eventually 

 break up into several divisions ; one branch goes haemally, 

 another neurally, and small branches go posteriorly and ante- 

 riorly, but the principal branch continues outward to one of the 

 six large m.arginal spines {a.s.'^-'i), which are attached to the sides 

 of the abdominal carapace. The branch from the first {JisiP) 

 branchial neuromere enters the first of these spines, where it 

 breaks up into fine fibers. The first five spines are innervated 

 by nerves from the corresponding five branchial neuromeres. 



f. Nerves from the Post-Branchial Neurofneres. — As there 

 are no appendages in the post-branchial metameres, and the 

 neuromeres are indistinguishably fused, it is difficult to follow 

 the metamerism in this region. In the typical neuromeres the 

 neural nerves supply the appendages exclusively, and the haemal 

 nerves the remainder of the metamere. In the post-branchial 

 metameres there are no appendages. If we consider the neural 

 nerves as absent, the post-branchial nerves fall into three similar 

 pairs, which partake very strongly of the nature of the typical 

 haemal nerves. 



( I ) Nerves from the First Post-Branchial Neuromere . — The 

 first post-branchial nerve (Pis. VI, VIII, and IX, Figs, i, 3-5, 

 h.7i.'*) is very similar to the last haemal branchial nerve. It 

 passes out from the fused, terminal, ganglionic mass posterior 

 to the muscles of the last gill and anterior to the last haemo- 

 neural muscle [h.n.m/-^), and goes diagonally backward toward 

 the margin of the carapace. Like the typical haemal nerve, 

 it is divisible into an intestinal branch, a post-cardiac branch, 

 and an integumentary branch. 



(a) Intestinal Branches. — In this neuromere the intestinal 

 branches {i.n.'*) arise sometimes at some distance from the gan- 

 glionic mass, and the muscular and visceral branches may arise 

 quite independently of each other (PI. VIII, Fig. 4). The 

 branch supplying the last haemo-neural muscle, which belongs 

 to this metamere, has not been found. 



(b) Post-Cardiac Bi'aiicJi. — The post-cardiac branch (s.c.n.'*) 

 arises at some distance from the proximal end of the nerve and 



