CRUSTACEANS. 605 



The middle membrane is formed of a layer of nucleated cells and 

 granular tissue, and of cells with fat-globules 1 . 



The blood-circulation, which was already more developed in 

 the highest arachnids, is here seen to be more and more perfected. 

 In all the heart is situated on the dorsal surface, and is arterial. 

 In the Omscutes, the Stomapods, and Limulus the heart has the 

 form of an elongated dorsal vessel, from which on both sides 

 branches arise, that are distributed to the different parts. In the 

 Lophyropoda the heart is more oval, and situated in the anterior 

 part of the body on the dorsal surface. In the ten-footed crus- 

 taceans (crays and crabs) the heart is roundish or hexagonal, much 

 broader than the main trunks of the arteries, and is situated on the 

 back between the third and fourth pairs of feet. The arteries in 

 the crustaceans are very differently disposed; in some there seem 

 to be only a few main trunks present, and the blood to flow else- 

 where merely in the interspaces of the organs in determinate direc- 

 tions, without being inclosed in vascular walls. In the ten-footed 

 crustaceans, where these vessels are best known, an artery arises 

 from the foremost part of the heart, in the middle, which is speci- 

 ally destined for the eyes ; and, besides some smaller branches 

 which it gives off, divides close to the eyes into two branches. 

 Next to this artery, there is one on each side which is distributed to 

 the antennae and neighbouring parts. More behind there arise, 

 about the middle of the heart, on its under side, two arteries, one 

 on each side, which run to the liver ; lastly, at the posterior ex- 

 tremity of the heart, there arises a considerable single artery, 

 which appears to fill the office of a posterior aorta, and gives off 

 branches to the intestinal canal, to the generative organs, to the 

 muscles of the abdomen, &c. AUDOUIN and MILNE EDWARDS 

 name this vessel sternal artery (artere sternale). It presently di- 

 vides into two main branches, of which one runs along the dorsal, 

 the other the abdominal surface. 



The veins in crustaceans, even in the most perfect order of the 

 Decapods, are represented by cells or spaces between the organs of 

 the body, in which the blood is moved without proper walls. 



1 See T. F. G. SCHLEMM De Hepate el Bile Crustaceorum et Mottmcorum quo- 

 nindam. Berolini, 1844, 4to. pp. 13 16 ; H. MECKEL in MUELLER'S Arcldv, 1846, 

 3538- 



