. The animans and man; an elementary textbook of zoology and human physiology. 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 th
. The animans and man; an elementary textbook of zoology and human physiology. 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. Theauricle receives the blood pouring from all the tissues ofthe body through the veins. It contracts and forces theblood into the ventricle. This then contracts and drives itinto a short vessel called the ventral aorta, which gives off abranch artery for each gill-arch. The gill-arteries divide. FIG. 22. Diagram of the circulatory system of a fish; v, ventricle; a,auricle. (After Parker and Haswell.) into capillaries in the gills, whence, after aeration, theblood is gathered by another artery and carried to the dor-sal aorta, from which branch arteries distribute it to thecapillaries of the general body-tissues. From these it isgathered by the veins and carried back to the auricle to be-gin again. In the course of circulation the blood reachesevery part of the body, picking up certain substances here,leaving others there, thus accomplishing the results alreadypointed out as the objects of the circulation. In the circulation of the higher vertebrates the moststriking difference from that of the fish is in the structure ofthe heart, which adapts the circulation to lungs instead ofgills, and in the more perfect control and regulation of theaction of heart and blood-vessels by the nervous system. It ma
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