oy anit 
CRUSTACEANS. 605 
The middle membrane is formed of a layer of nucleated cells and 
granular tissue, and of cells with fat-globules!. 
The blood-cireulation, 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 Oniscides, 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 antenne 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 Epwarps 
name this vessel sternal artery (artére 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 perfeet 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. Scunemm De Hepate et Bile Crustaceorum et Molluscorum quo- 
rundam. Berolini, 1844, 4to. pp. 13—16; H. Mrcxer in Mururr’s Archiv, 1846, 
8. 35—38. 
