196 PATTEN AND REDENBAUGH. 



EXPLANATION OF PLATE IX. 



Fig. 5. The heart of Limulus, with adjoining organs and nerves, seen from 

 the haemal side (drawn to same scale as Figs. 3 and 4). 



The haemal side of the carapace has been stripped off and the epidermis and 

 median inter-tergal muscles removed in order to show more clearly the heart 

 and arteries. The nerves, however, lying within these omitted portions are 

 represented. 



The heart (Jit) lies in the pericardial sinus, which is indicated by the shaded 

 area upon each side of the organ. Eight pairs of ostia (ps!^''^^) are seen upon the 

 haemal side of the heart, and the median (in.c.n.) and two lateral cardiac nerves 

 {l.c.n.) are indicated. The striated appearance of the walls of the heart is due to 

 the longitudinal strands of connective tissue. 



Upon the left side of the heart the alary muscles {al.m.^'^^) are represented, 

 but upon the right they are omitted in order to show the four lateral arteries 

 {l.ary^) and the positions of the collateral arteries {car.), which unite posterior 

 to the heart to form the superior abdominal artery {s.a.ar.). Another lateral 

 artery {l.ar.^) sometimes arises from the anterior end of the heart in fronT: of the 

 aortic valve. Upon the outer sides the collateral arteries give off numerous 

 branches to the muscles and haemal side of the body, and upon the median sides 

 they give branches to the intestine, the superior intestinal branches {s.i.ar}^). The 

 aortic arches {ao.a.) and frontal artery {/.ar.), running over the proventriculus 

 (J>rov.), proceed from the anterior extremity of the heart. 



Five pairs of branchio-cardiac canals bring blood to the pericardial sinus ; the 

 first of these is formed of two canals, one (i.c.c.^) from the operculum and the 

 other (d.c.c.'^) from the first gill. The remaining four pairs of branchio-cardiac 

 canals {d.c.c.^°'^^) bring the blood from the last four pairs of gills. 



The seven pairs of entapophyses {eniaJ''^*) are represented, and the prin- 

 cipal muscles attached to the haemal side of the carapace ; three tergo-proplastral 

 muscles {t.p.m.^-c), two slips of the branchio-thoracic muscles (b.t.m.^ ^^'^ ''), the 

 dorso-lateral-plastro-tergal muscle {d.l.p.t), and a slip of the inter-tergal muscle (i.m.) 

 attached to the first entapophysis (enta.^ ''"'^ ^) are represented in the cephalo- 

 thoracic region ; in the abdominal region are seen the haemal ends of the seven 

 pairs of haemo-neural muscles (/t.fi.m.^'^*) and the six external branchial muscles 

 {e.S.m.^'^^) of the right side, the levator ani (/.a.) and the extensors {t.e.m.^ ^^^ '') 

 of the telson. One slip of the extensor {t.e.niA) is attached to the last three 

 pairs of entapophyses {ettta.^-'^'). 



The end of the median eye nerve {m.ey.n.) appears in front of the proven- 

 triculus. 



Seven pairs of segmental cardiac nerves (j.ir.;/.^"'^) and two post-cardiac nerves 

 {s.c.n.^* ^"'^ ^5) come from the neural side of the body and pass up into the epider- 

 mis haemal to the heart. The most anterior one (s.c.n.^) supplies the omitted 

 inter-tergal muscle, and also a portion of the epidermis. The next one {s.c.nJ ^"'^ 8) 

 is formed of the fused branches of the seventh and eighth neuromeres ; this 

 also innervates the above-mentioned inter-tergal muscles and sends branches to 

 the epidermis in the median line; one branch, the pericardial nerve (/•«■), goes 

 posteriorly in the epidermis, and gives a branch to each of the branchial cardiac 



