62 



THE ANIMALS AND MAN 



dilatory system is found to be highly developed and to 

 work with great efficiency. 



In Amceba, because of its small size and the constant 

 flowing of the body-substance, there is no circulatory system. 

 In some Protozoa the contents of the body-cell seem to have 

 a definite movement, but there are no such organs as heart 

 and blood-vessels. In most animals we 

 find blood and a system of tubes and 

 spaces for it to circulate in. In some, 

 as the insects (fig. 21), only part of the 

 circulatory system consists of definite 

 tubes; these open into loose ill-defined 

 spaces in the body-cavity. In these 

 spaces the blood moves gradually 

 throughout the animal, but not so 

 definitely and quickly as in others 

 where the blood runs in definite vessels. 

 In the earthworm there is no "heart" 

 as in higher animals, but the blood- FIG. 21. Diagram of 

 vessel along the dorsal line and some circulatory systern of 

 of its branches around the sides have 

 muscular walls and "beat" by a wave 



of contraction running toward the 



a young dragon-fly; 

 in the middle is the 

 chambered dorsal 

 vessel with single 

 artery; the arrows 

 indicate the direction 

 of blood - currents. 



head. In insects the dorsal blood- 

 vessel beats in the same way, but 

 generally more vigorously. In the young 

 larva of a mosquito or nymph of a May-fly with transparent 

 skin the beating can be easily seen under the microscope. In 

 molluscs there is a well-developed heart; it can be well seen 

 in the fresh-water mussel. The crustaceans also have a heart. 

 This can be seen at work in a water-flea under the micro- 

 scope, or can be readily demonstrated in a crab or crayfish 

 killed with chloroform or ether. 



In vertebrates the blood circulates in a definite system of 

 tubes through which it is pumped by a heart. The fishes 



