ASTACUS FLUVIATILIS. 283 



really is), are wholly unconnected with the heart, which thus 

 is, in a manner, suspended freely in the blood. 



Six apertures, two of which are superior, two inferior, 

 and two lateral, provided with valves which open inward, 

 allow the blood to enter the cavity of the heart during the 

 diastole, and prevent its egress, except by the. arteries, dur- 

 ing the systole. The arterial trunks are six in number, five 

 being given off anteriorly, and the other from the posterior 

 portion of the heart. 



Of the five anterior arteries, one, the ophthalmic, is single, 

 and situated in the middle line ; it passes forward on the 

 stomach to the head, where it supplies the eyes and anten- 

 nules. The other arteries are in pairs ; two pass on the 

 stomach forward and outward, giving off branches to the 

 carapace, and eventually supplying the antennae ; the other 

 two pass downward, between the anterior lobes of the geni- 

 talia, and divide into a multitude of branches upon the he- 

 patic casca. 



The posterior trunk, or sternal artery, is the largest of 

 all, and presents a sort of bulbus arteriosus at its commence- 

 ment. It turns almost directly downward, usuallv on the 

 right side of the intestine, to the sternal canal, which it 

 enters, passing between the antepenultimate and penultimate 

 thoracic ganglia to the lower surface of the ganglionic cord ; 

 it gives off two abdominal branches, one superior, close to its 

 origin from the heart, which traverses the middle of the ter- 

 gal region above the intestine, the other inferior, which takes 

 a corresponding course along its sternal region beneath the 

 nervous system. The arterial trunks are provided with valves 

 at their commencement, so arranged as to prevent the regur- 

 gitation of the blood. They ramify minutely, but how far a 

 capillary system can be said to exist, is a question requiring 

 further investigation. In transparent Zocza?, 1 have plainly 

 observed the abrupt termination of the arterial trunks by 

 open mouths, through which the blood was poured into wall- 

 less lacuna?, and into the general cavity of the body ; nor can 

 there be the least doubt that a similarly lacunar condition 

 of the circulation exists in those lower adult Crustacea, the 

 transparency of which allows of their examination with the 

 requisite powers of the microscope. The probability is that 

 a similar state of things obtains in the vascular system of all 

 other Crustacea, and that, after undergoing a greater or less 

 amount of subdivision, the arterial vessels, or their capillary 

 continuations, cease to exist, the blood then making its way 



