SANGUIFEROUS SYSTEM. 453 



extremities, and provided with terminal valves ; they present 

 narrow lateral openings at their dilated extremities, and they 

 give off symmetrical branches on each side. They constitute 

 a wide fusiform sac tapering much at its extremities, and 

 extended along the middle of the back from the proximal 

 caudal segment to the third pair of legs ; they are connected 

 with the internal organs by lateral vessels, especially with 

 the hepatic tubuli, the fatty substance, and the pulmonary 

 sacs ; and the internal terminal valves produce an articulated 

 appearance of the heart externally, as observed by Trevira- 

 nus. As in other entomoid animals, the anterior part of the 

 thick muscular opaque heart terminates suddenly in the thin 

 membranous transparent aorta which appears to widen before 

 dividing into lateral branches, and the small posterior com- 

 partment of the heart receives the great median dorsal vessel 

 of the caudal segments. The anterior distribution and lateral 

 arches of the aorta nearly resemble those of the scolopen- 

 dra, and the blood appears to be returned backwards chiefly 

 by the epineural vein, accompanying the moto-sensitive co- 

 lumns, as commonly seen in myriapoda, Crustacea, and 

 other articulated animals. 



The higher crustaceans animals exhibit a more perfect 

 development of the sanguiferous system, in the structure of 

 the heart, the fibrinous character of the blood, and the extent 

 of the arterial and venous distribution, than is presented by 

 any other of the articulated classes ; but in the lower forms 

 of these animals, as in isopoda and stomapoda, the circula- 

 tion more resembles that of annelides and the larvee of insects, 

 especially in the elongated and simple form of the dorsal 

 vessel, and the absence of capillary ramifications. From the 

 defined nature of the respiratory organs of Crustacea, and the 

 imperfect aeration afforded by the medium in which they 

 live, the double circulation of these animals, like that of the 

 pulmonated arachnida, is more complete than in other articu- 

 lata, and the heart is systemic as in almost all the inverte- 

 brated tribes. The venous blood in the decapods, as in the 

 maja squinado, (Fig. 1 33. E.) is collected from the various 

 members by distinct brachial veins, (E. a. a.) and from the 

 upper and lower parts of the trunk by a series of superior 

 (E. c.) and inferior (E. b.} abdominal veins, which convey it 

 to capacious lateral sinuses connected freely with each other, 

 and situate at the bases of the several branchiee. The bran- 



