DEVELOPMENT OF LIMULUS POLYPHEMUS. 171 



is a long fusiform tube (as seen in the living animal), lying immediately over the alimentary 

 canal, and rather shorter than the latter. It beats ninety times a minute, after the first 

 moult. The anterior end is rounded (the origin of the main artery not being in view), 

 with a slight constriction just behind, while the posterior end gradually diminishes in width, 

 ending at a little distance in front of the anus. There are seven pairs of venous openings 

 guarded by valves, the venous openings being vertical slits, alternating slightly, one open- 

 ing a little in advance of the one opposite. Those in the cephalothorax are larger than the 

 abdominal ones, gaping wider. There is a wider interval between the second thoracic and 

 the first abdominal pairs of venous openings than between the others. I could not detect 

 the pericardium, but could see the blood globules moving to and fro in the pericardial cav- 

 ity. Straus Durckheim,^ Van der Hoeven,^ and Owen,^ describe six arteries, three of which 

 are sent off from the head, and three from the posterior end of the heart. I am not aware 

 that they have noticed an additional pair thrown off from near the middle of the heart, and 

 just in front of the hinder end of the cephalothorax. My attention was called to them by 

 a steadily flowing stream of blood discs flowing away from the heart, at right angles to it. 

 No venous openings in the heart could be seen at this place. In the figure (PI. v, fig. 27), 

 the course of the arteries is represented by feathei'ed arrows ; the venous currents by sim- 

 ple arrows. Authors describe the anterior end of the heart as terminating in an aortic 

 trunk, Avhich throws off three arteries ; a median one towards the ocelli, and a lateral 

 pair of larger ones which curve down under the oesophagus. I could not see the walls of 

 any of the arteries, and indeed the arterial blood seemed to flow in channels exactly like 

 the venous sinuses, as in the arteries which pass around the margin of the carapace the 

 blood discs were seen to pass by irregular cui-rents towards the front edge of the margin. 

 The anterior aorta could not be detected in the young Limulus, but on each side of the end 

 of the heart the blood could be seen rushing out and in, and with a general course down- 

 wards, beneath the oesophagus, while a current of blood flowed on each side of the stomach 

 and oesophagus, and thence went out at a considerable angle to the edge of the carapace, 

 where it divided, sending a branch around under the ocelli, and another along the outer 

 edge of the cephalic shield, and again subdivided opposite the second pair of cardiac 

 valves, one current following the edge of the cephalothorax, and the other going in towards 

 the heart. The marginal artery continues around the edge of the abdomen, the current of 

 blood turning upon itself before reaching the caudal spine, and flowing towards the heart. 

 The abdominal arteries, represented by powerful currents of blood issuing from between 

 the two last pair of cardiac valves, are directed obliquely outwards and backwards. The 

 caudal aorta sends a current nearly to the tip of the spine, the venous sinuses returning it 

 along the sides. The simple arrows mark the course of the returning currents, which flow 

 from all parts of the body towards the valves. 



In the legs a current passes towards the claws along the inner side, and back by the 

 outer side ; the main current crosses over to the base of the larger claw, sends a small 



^Considerations generates sur rAnatomie comparee des the Invertebrate Animals. London, 1843. (I might add that 



Animaux articules. Strasbourg, 1828. the observations here recorded were made on the living an- 



^Recherehes sur I'Histoire Naturelle et TAnatomie des imal, without the aid of dissection, and do not intend to 



Limules. Lej-den, 1838. give the entire anatomy of the circulatory system, hut that 



Lectures on the Comparative Anatomy and Physiology of of those parts only that may be seen from a dorsal view.) 



