106 
COMPAKATIVE ZOOLOGY. 
extending from tail to liead above the alimentary canal, 
a similar tube on the ventral side through which the blood 
returns, and cross-tubes in every segment. In the Lob¬ 
ster and Crab, Spider and Scorpion, the dorsal tube sends 
Fig. 70.—Circulation in a Lobster: a , heart; 6, artery for the eyes; c, artery for an¬ 
tennae; d, hepatic artery; e, superior abdominal artery; /, sternal artery; g, ve¬ 
nous sinuses transmitting blood from the body to the branchiae, h, whence it 
returns to the heart by the brauchio-cardiac vessels, i. 
off a system of arteries (not found in Insects); but the 
blood, as it leaves these tubes, escapes into the general 
cavity, as in other Arthropoda. The Lobster and Crab, 
however, show a great advance in the concentration of 
the propelling power into a short muscular sac. 
A third development of the circulatory system is fur¬ 
nished by the Mollusks. Comparatively sluggish, they 
need a powerful force-pump in the form of a compact 
heart. In the Oyster and Snail (Figs. 44,45), we find such 
an organ having two cavities—an auricle and a ventricle, 
one for receiving, and the other for distributing, the blood. 
The auricle injects the blood into the ventricle, which 
propels it by the arteries to the various organs. Thence 
it passes, not immediately to the veins, as in higher ani¬ 
mals, but into the spaces around the alimentary canal. A 
part of this is carried by vessels to the gills or lung, and 
then returned with the unpurified portion to the auricle. 
The whole of the blood, therefore, does not make a com¬ 
plete circuit. The Clam has a similar heart, but with two 
auricles. 
