THE CIRCULATION OF THE BLOOD 311 



into the dorsal tube through the valvular openings as it 

 expands ; and upon its contraction, all the side valves 

 are closed, and the fluid is forced toward the head. 

 Passing out at the front opening, it is again diffused 

 among and between the tissues of the body. The 

 blood, therefore, does not describe a circle in definite 

 channels so as to return constantly to its point of 

 departure. 



Many worms (as the earthworm) have a pulsating 

 tube extending from tail to head above the alimentary 

 canal, a similar tube on the ventral side through which 



FIG. 267. Circulation in a Lobster: a, heart; 3, artery for the eyes; c, artery for an- 

 tennae; d, hepatic artery; e, superior abdominal artery: f, sternal artery; g, venous 

 sinuses transmitting blood from the body to the branchiae, h, whence it returns to the 

 heart by the branchio-cardiac vessels, i. 



the blood returns, and cross tubes in every segment 

 (Fig. 52). In the lobster and crab, spider and scorpion, 

 the dorsal 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 t>ther 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 

 furnished by the mollusks. Comparatively sluggish, 

 they need a powerful force pump in the form of a com- 

 pact heart. In the oyster and snail (Figs. 242, 243), 

 we find such an organ having two cavities an auricle 



