No. I.] STUDIES ON LIMULUS. 131 



Anteriorly the collateral arteries give off branches to the 

 tergo-proplastral and tergo-coxal muscles, and to the intestine. 

 Opposite the second pair of ostia a large pair of arteries are 

 given off, which pass outward toward the posterior angles of 

 the cephalothorax and anastomose with the anterior marginal 

 arteries. Midway between the median line and the edge of the 

 carapace a large branch, the hepatic artery, is given off ante- 

 riorly, supplying the liver and anastomosing with the lateral eye 

 arteries. At the same point a posterior marginal artery is 

 given off. This passes posteriorly along the margin of the 

 abdominal carapace and anastomoses with the superior abdom- 

 inal artery at the base of the caudal spine. 



The posterior portions of the collateral arteries send branches 

 to the intestine, to the external branchial muscles, and to the 

 muscles of the caudal spine. The arte^'ia abdo^ninalis superior 

 {s.a.ar.) gives off branches to the haemal side of the intestine, 

 anastomoses with branches from the ventral artery and poste- 

 rior marginal artery, and terminates in the caudal spine. All 

 the arteries divide ultimately into very fine arterioles, which 

 open into venous spaces. 



c. Venous System. 



There are no veins with definite walls lined with epithelium. 

 The venous system consists, for the most part, of tubular spaces 

 in the areolar tissue, or irregular spaces between the various 

 organs. The blood is collected from these spaces into a pair 

 of longitudinal sinuses (PI. IX, Fig. 6, v.c.s.) upon the neural 

 side of the body. These sinuses have already been described 

 as spaces, between the branchio-thoracic muscles and the 

 longitudinal abdominal muscles, roofed in on the haemal side 

 by a membrane which furnishes attachment for the veno-peri- 

 cardiac muscles. From these venous sinuses the blood passes 

 into the operculum and the lamellae of the five pairs of gills. 

 From each gill a large branchio-cardiac canal (PI. IX, Figs. 5 

 and 6, b.c.cP-'^) carries the blood to the pericardial sinus {p.s). 

 A canal {b.c.cF) from the operculum unites with the branchio- 

 cardiac canal {b.c.cP) of the first gill. There are, therefore, 



