. The animans and man; an elementary textbook of zoology and human physiology. emoved. In such animals the cir- 62 THE ANIMALS AND MAN dilatory system is found to be highly developed and towork with great efficiency. In Amceba, because of its small size and the constantflowing of the body-substance, there is no circulatory some Protozoa the contents of the body-cell seem to havea definite movement, but there are no such organs as heartand blood-vessels. In most animals wefind blood and a system of tubes andspaces for it to circulate in. In some,as the insects (fig. 21), only part of
. The animans and man; an elementary textbook of zoology and human physiology. emoved. In such animals the cir- 62 THE ANIMALS AND MAN dilatory system is found to be highly developed and towork with great efficiency. In Amceba, because of its small size and the constantflowing of the body-substance, there is no circulatory some Protozoa the contents of the body-cell seem to havea definite movement, but there are no such organs as heartand blood-vessels. In most animals wefind blood and a system of tubes andspaces for it to circulate in. In some,as the insects (fig. 21), only part of thecirculatory system consists of definitetubes; these open into loose ill-definedspaces in the body-cavity. In thesespaces the blood moves graduallythroughout the animal, but not sodefinitely and quickly as in otherswhere the blood runs in definite the earthworm there is no heartas in higher animals, but the blood- FIG. 21. Diagram ofvessel along the dorsal line and some circulatory systern ofof its branches around the sides havemuscular walls and beat by a wave. of contraction running toward the a young dragon-fly;in the middle is thechambered dorsalvessel with singleartery; the arrowsindicate the directionof blood - currents. head. In insects the dorsal blood-vessel beats in the same way, butgenerally more vigorously. In the younglarva of a mosquito or nymph of a May-fly with transparentskin the beating can be easily seen under the microscope. Inmolluscs there is a well-developed heart; it can be well seenin the fresh-water mussel. The crustaceans also have a can be seen at work in a water-flea under the micro-scope, or can be readily demonstrated in a crab or crayfishkilled with chloroform or ether. In vertebrates the blood circulates in a definite system oftubes through which it is pumped by a heart. The fishes ANIMAL PHYSIOLOGY 63 (fig. 22) have the heart consisting of two parts, with mus-cular walls, a single auricle and a single ventricle. Theauric
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