^No. I.] STUDIES ON LIMULUS. 175 



arrangement is most typical in the abdominal region, and it will 

 be well to take up one of these neuromeres first. 



In the first gill neuromere (Text-fig. 8; Pis. VIII and IX, Figs. 

 3, 4, and 6) two small nerves arise very close together from the 

 haemal side of the haemal nerve iji.n.^) close to the abdominal 

 ganglion. The anterior one {a.i.71.) enters the adjacent mass of 

 longitudinal abdominal muscles and communicates with the 

 plexus supplying these muscles. The posterior one (p.i.ii.) 

 divides into two branches, one going to the haemo-neural muscle 

 (Ji.n.my) and the other to the intestine. The latter has been 

 observed to communicate with the corresponding nerves of other 

 neuromeres by a plexus lying in the tissue immediately surround- 

 ing the intestine. Only glimpses of this plexus have been 

 obtained here and there. The nerves are very fine and not 

 easily made out without a microscope. In one or two instances, 

 while working with methylen blue, a fine plexus was seen in the 

 tissues haemal to the intestine, near the posterior end. 



In the tenth, eleventh, and twelfth neuromeres the roots of the 

 two intestinal nerves arise a little farther apart. In each case 

 the anterior nerve joins the plexus in the longitudinal muscles. 

 The posterior nerve gives a branch to the haemo-neural muscles, 

 and one to the intestine, and also communicates sometimes 

 with the plexi in the longitudinal abdominal muscles, and in the 

 tissues surrounding the intestine. 



In the neuromeres posterior to the twelfth the intestinal 

 nerves are very irregular in their origin, sometimes arising 

 from the haemal nerve at some distance from the ganglion, and 

 entering the intestine by several branches. In the specimen 

 represented in Fig. 4 of the plates the posterior branch of the 

 intestinal nerve {i.nP) of the twelfth neuromere traverses the 

 twelfth haemo-neural muscle [h.n.m/^) before entering the in- 

 testine. In this figure the branches going to the intestine are 

 represented as cut off. 



The next intestinal nerve {i.n.'^) has no muscular branch. 



The fourteenth (i.n."') upon the right side arises near 

 the abdominal ganglion and proceeds posteriorly a long dis- 

 tance, and finally enters the intestine by three branches. 

 Upon the left side the fourteenth intestinal nerve {i.nJ*) is 



