THE HYPO-BRANCHIAL MUSCLES AND NERVES. 105 



(Figs. 57,59, Lab.) The fibers arise from small branches of the sixth to the sixteenth 

 haemal nerves. The branches are given off from the anterior side of the haemal 

 nerve, close to the cord. The neurones lie on the opposite side of the next anterior 

 neuromere, with those that supply the branchio-thoracic muscles. (Fig. 60, D,l.ab.) 

 In the seventh and eighth metameres, the plexus appears to be continuous with 

 that going to the intestine. 



The General Cutaneous Nerves are largely, if not wholly, sensory. They 

 extend over the surface of the branchial plastron, dividing into numerous branches 

 on the margin. (Figs. 59, 70, g.cut.) The fibers that enter into these branches 

 probably form the second root, h.r. 2 (Fig. 61.) 



Cardiacs. For a description of the segmental cardiacs, see p. 200. 

 The Branchio-thoracic, or Hypo-branchial Muscles and Nerves. 



The hypo-branchial muscle is a large compound muscle derived from the 

 eighth to thirteenth metameres inclusive. The neural end of each component is 

 separate and terminates in a tendinous infolding of the ectoderm at the base of 

 its corresponding appendage. The haemal ends form a single massive muscle 

 which shifts its position a long ways forward into the haemal region of the thorax, 

 where it is attached to the inner surface of the shield, in front of the anterior end 

 of the heart and the forebrain. (Fig. 78, B.) 



The muscle aids in the performance of the complicated respiratory move- 

 ments, drawing the bases of the gills forward and upward; it also aids in flexing 

 the thorax on the branchial section of the body. 



The hypo-branchial nerve 1 forms a great longitudinal trunk extending over 

 the neural surface of the muscle. (Figs. 59 and 77, b.th.n.) It receives its fibers 

 from the eighth to the fourteenth haemal nerves, via short communicating 

 branches. It is, therefore, to be regarded as a compound nerve formed by the 

 united branches of at least seven segmental nerves. For the greater part of its 

 course, it forms a compact longitudinal trunk, giving off at regular intervals 

 branches to the proximal ends of the muscle slips, near their tendinous at- 

 tachment to the base of the gills. At its anterior end, it breaks up into many 

 branches that are distributed through the single muscle into which the six 

 separate muscles merge. One nerve separates from the anterior end of the main 

 trunk and supplies the inter -tergal, or arthro-tergal, muscle. (Fig. 77, in.t.) 



The nerve fibers arise from clusters of large D neurones lying on the opposite 

 side of the cord, on the posterior margin of the neuromere, in front of the one 

 where the nerve enters the cord. (Fig. 60.) The very large axones cross in the 

 posterior haemal commissure and pass backward, as a conspicuous bundle of 

 large nerve tubes, A.r. 5 , to the haemal nerve root. Some of the fibers go to the 

 longitudinal abdominal nerves, but the main bundle passes on with the haemal 

 nerve, leaving it farther on, to enter the main hypobranchial. 



The remarkable condition of the hypobranchial muscles and nerves of 

 Limulus is, no doubt, one that has its counterpart in other arachnids. At present, 



1 Lateral Sympathetic of Patten and Redenbaugh. 



