No. I.] STUDIES ON LIMULUS. 151 



neuromeres the intestinal and cardiac branches are absent. 

 The sixth neuromere, however, contains the typical number of 

 branches. 



In all cases the haemal nerve (Text-figs. 1 1 and 12 ; Pis. VI- 

 VIII, and X, Figs. 1-3, 11, and 12, //.;;.) arises from the haemal 

 side of the brain slightly anterior to the neural nerve of the 

 same neuromere. It is about half as large as the neural nerve, 

 does not have a ganglionated base, and is not accompanied by 

 an artery except for a short distance from its origin. It is pe- 

 culiar in having at a greater or less distance from its origin a 

 ganglion-like swelling. This swelling, which has been described 

 by Milne-Edwards, contains, however, no ganglion cells, but the 

 fibers at these points undergo a complicated interlacing, and 

 numerous nuclei are present. 



(a) Intestinal Brandies. — The intestinal branch is absent in 

 the third, fourth, and fifth neuromeres. In the sixth (Text-fig. 

 12; Pis. VI-VIII and X, Figs. 1-4, 11, and 12, i.n!') it 

 arises from the haemal side of the haemal nerve and passes 

 between the plastro-coxal muscles of the fifth and sixth legs, 

 through a foramen (PI. VIII, Fig. Af,fP) in the endochondrite, 

 into the longitudinal abdominal muscles attached to the endo- 

 chondrite. Here it communicates with a plexus which supplies 

 these muscles. Some of the branches, however, pass on to the 

 intestine. In the third, fourth, and fifth metameres the intestine 

 is supplied by nerves which pass forward from the plexus in the 

 longitudinal abdominal muscles. Some of the nerves have also 

 been traced from this plexus to the tergo-plastral and veno-peri- 

 cardiac muscles in this region. 



In the second metamere a nerve (Text-fig. 13 ; Pis. VIII and X, 

 Figs. 3, II, and 12, i.n.^) is given off from the haemal nerve 

 (h.n/) close to its base. It passes haemally upon the median 

 side of the anterior cornu (a.cor.), and supplies the tergo-proplas- 

 tral muscles {t.p.m."-'^'^^^). No branch has been observed going 

 to the intestine, but as it supplies muscles similar to those sup- 

 plied by the other intestinal branches, and also has an origin 

 similar to that of the other intestinal branches, it has been 

 included in the same category. 



(b) Cardiac Branch. — The cardiac branch (Text-figs. 12 



