CRUSTACEA. 319 



off {k, k) the hepatic arteries. The larger posterior chamber receives 

 the two lateral venous apertures, and gives off (Ji) the superior caudal 

 arterj, and (i) the sternal artery, which is the chief vessel, and the 

 only one which has a pair of semilunar valves at its origin. 



A portion only of the ordinary venous blood is returned directly 

 by the four valvular orifices just described. The blood is returned 

 from the maxillae and legs to a series of inferior lateral sinuses at the 

 bases of the branchiae ; from these sinuses it passes, by vessels per- 

 forming the office of branchial arteries, along the outer margin of the 

 gill, and after being distributed over the branchial lamellae, is collected 

 into the vein along the inner margin of the gill, and by the union of 

 the branchial veins into one trunk, on each side, is poured into the 

 ventricle by the two orifices on its under surface (</, d). 



We may trace in the heart of the Crustacea a gradational series of 

 forms, from the elongated median dorsal vessel of the Limulus, to the 

 short, broad, and compact m-jscular ventricle in the lobster and the 

 crab (Jig. \Z2,f). In all the Crustacea, as in all the other articulate 

 animal*, the heart is situated immediately beneath the skin of the 

 back, above the intestinal tube, and is retained in situ by lateral 

 pyramidal muscles (Jig. 133, a, a). In the Entomostraca, and in the 

 lower, elongated, slender, many-jointed species of the Edriophthalmous 

 Crustacea, the heart presents its vasiform character : it is broadest 

 and most compact in the crab. 



In this series we may trace a general correspondence in the pro- 

 gressive development of the vascular as of the nervous system, con- 

 comitant with the concentration of the external segments, and the 

 progressive compactness in the form of the entire body. But there 

 is a remarkable exception to this concomitant progress in the Limulus, 

 indicative, with the general condition of the instruments of locomo- 

 tion and respiration, of the essentially inferior grade of organisation 

 of that genus, which, as has already been observed, seems to be the 

 last remnant of the once extensive group of Trilobitic Crustacea, 

 which swarmed in the seas of the ancient secondary periods of the 

 earth's history. 



We have seen in the compact and broad existing representative of 

 those extinct gigantic Entomostracans, that the nervous system ex- 

 hibits a concentration of its principal central mass around the mouth, 

 but with a ganglionic double cord continuing from it. The heart, 

 however, is far from presenting a corresponding degree of concentra- 

 tion ; it remains an elongated fusiform tube, extending parallel with 

 the intestine from the pylorus to the rectum, like that of the Squillce : 

 in Limulus. it is contained in a pericardium with thin membranous 

 walls, formed by the central sinus of the venous system, and it receives 



