THE BLOOD AND ITS CIRCULATION. 



467 



fluid finds its way backward through the lacunie or passages between 

 the tissues and viscera. The dorsal vessel prevents the stagnation of 

 the fluid. 



In the myriapods the dorsal trunk has as many segments as there 

 are joints of the body. One of the millepeds has not less than one 

 hundred and sixty. Centipeds have generally twenty-one segments, 

 and besides the pair of valves for each joint there is given off a pair 

 of arteries. These unite to form a ventral tube. Insects have the 

 heart segmented only in the abdomen, and never more than eight 

 segments. An arterial prolongation of the trunk as a simple tube ex- 

 tends to the head. 



As spiders and scorpions have localized breathing-sacs, the}^ re- 

 quire a respiratory circulation. This 

 is secured, not by special tubes, but 

 by the passage of the blood, on its 

 return to the heart, through venous 

 sinuses or special passages between 

 the internal organs. 



The best heart among articulates 

 is possessed by the crustaceans, the 

 largest, though not the highest, ani- 

 mals of the sub-kingdom. Crabs and 

 lobsters have a concentrated heart, a short, fleshy sac, with great pro- 

 pelling power, which sends the blood by several branching arteries to 

 the parts of the body. AVe now find a concentration of the power 

 which had been previously diffused in a long tube. 



Fig. 9. Diagram OF Mollusk. a, Alimentary 

 Canal ; h^ Heart ; , r/, n'\ Nervous 

 Gauglia. 



Fig. 10. Cross-sectional Diagram of a Fresh-water Mussel. /, Ventricle ; g. Auricles ; c, 

 Rectum ; p, Pericardium ; h, i, Gills ; B, Foot ; A, A, Mantle or Skin. 



As the mollusks are mostly so sluggish that their circulation has 

 little aid from the movements of the body, they require a more pow- 



