106 COMPARATIVE ZOOLOGY. 



extending from tail to head above the alimentary canal, 

 a similar tube on the ventral side through which the blood 

 returns, and qross-tubes in every segment. In the Lob- 

 ster and Crab, Spider and Scorpion, the dorsal tube sends 



Fio. 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 branchio-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 un purified 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. 



