128 PATTEN AND REDE ABA C/GH. [Vol. XVI. 



The posterior portion receives the five pairs of canals {b.c.c.'^'^) 

 from the gills. 



In normal specimens there are eleven arteries given off from 

 the heart, three from the anterior extremity, and four pairs 

 from the sides, opposite the four anterior pairs of functional 

 ostia. The two large anterior arteries, aortic arches (Pis. VIII 

 and IX, Figs. 3, 5, and 8, ao.a.), curve downward, one on each 

 side of the proventriculus, to the circum-oesophageal collar. 

 The median artery, arteria frontalis [f.ar.), goes directly 

 forward over the haemal surface of the proventriculus. A 

 large pocket-shaped valve (PI. IX, Fig. 8, a.v.), much like a 

 vertebrate semilunar valve, lies upon the haemal wall of the 

 heart at the base of the aortic arches, and just behind the rudi- 

 mentary ostia. It prevents a backward flow of the blood from 

 the aortic arches and from the arteria frontalis. 



The lateral arteries, arteriae lateralcs (Pis. VIII-X, Figs. 

 3, 5, 8, and 9, l.ar!^~^), arise from the lateral, neural corners of 

 the heart, directly beneath the four anterior pairs of functional 

 ostia {psfi~f), and pass downward into the pericardium, and out- 

 ward to a pair of longitudinal collecting arteries, the arteriae 



collaterales ic.ar). The arteriae col- 

 lateralcs (PI. IX, Figs. 5, 6, and 8, 

 c.ar)^ pass backward, giving off nu- 

 merous branches, and unite behind 

 the posterior end of the heart to form 

 a median artery, the arteria abdotni- 

 ^Ti.w: jialis superior {s.a.ar.), which appears 



Fig. 7. — Diagram showing the mechan- , i j- ^, , . 



ism of the valves of the ostia. h.w., tO prOCCCd frOm the pOStCriOr ex- 

 haemal wall of heart; n.w., neural t^emitv of the heTrt 



wall of heart; 0.5., ostium of heart; Lremuy oi me neart. 



^r., grating of elastic fibers upon the j^g artcriae Itttcralcs arc supplied 



outside of the heart, bridging over ^ '- 



the ostium, and preventing the lips with paired Semilunar valvcs (PI. IX, 



of the ostium from spreading apart; ... r • 



>«.:?., muscle fibers attached to the lips Fig. 8, S.V.^) at thCir pOmtS of Ori- 

 of the ostia at the outer comers. . r .^■^ . t-i i 



gm from the heart. Ihese valves 

 are upon the posterior and anterior walls of the arteries. 



Each of the ostia is also provided with a valve, the action 

 of which is best seen in young Limuli. The lips of the 

 ostia (Text-fig. 7, os.) turn inward toward the lumen of the 

 heart, and this invagination is greatest at the outer corners. 



