THE CIRCULATION OF THE BLOOD. 105 
channels so as to return constantly to its point of de- 
parture. 
Certain Worms (as the Earth-worm) have a ventral 
tube, conveying the blood from head to tail, with numer- 
ous cross canals joining the ventral and dorsal tubes. In 
the Lobster and Crab, Spider and Scorpion, the dorsal 
Fig. 69.—Circulation in a Lobster: a, heart; 6, artery for the eyes: c, artery for an- 
tenn; d, hepatic artery; e, superior abdominal artery ; f, sternal artery; g, ve- 
nous sinuses transmitting blood from the body to the branchiw, h, whence it 
returns to the heart by the branchio-cardiac vessels, 7. 
tube sends 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 Articulates. The Lobster and 
Crab, however, show a great»advance in the concentration 
of the propelling power into a short muscular sac—the 
first rudiment of a true heart. 
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, 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 countless arteries to the various organs. Thence it 
passes, not immediately to the veins, as in higher animals, 
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 
