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Page:Popular Science Monthly Volume 49.djvu/197

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that the frog is devoid of any neck; that its spine makes a hump near the middle of its back; the characters of its two pairs of limbs; and other external features, are all too well known to the intelligent observer and reader to require special description here. There is another thing we must notice, however, and its presence is not generally observed nor appreciated. If we watch carefully at the distal end of the backbone, upon either side of it, it will be seen that the skin pulsates at those points with sufficient force to make it apparent to the eye. These pulsations are performed by the posterior pair of lymph hearts. Now, the lymph hearts have nothing to do with the circulation of the blood as performed by the heart, but they, on the other hand, pump the lymph contained in the large lymphatic vessels into the veins. There are two pairs of these lymph hearts—the pair just noticed and an anterior pair, which are below the margin of the shoulder blade, upon either side, and near the lateral processes of the third vertebra. They are muscular organs endowed with the power of contraction, and are extremely important ones in the internal economy of the frog.

In a brief essay, such as I am now writing, it will by no means be practicable to enter upon the extremely interesting subject of the internal structure of the frog. Even to touch upon this ever so lightly would require a small volume to print it. Not a few books are in circulation now devoted largely to the anatomy of these animals, and others no doubt will appear from time to time. A few years ago the distinguished British naturalist, Prof. St. George Mivart, devoted an entire treatise to The Common Frog, and it is truly a most instructive work. In it he describes a number of different kinds of frogs, but what gives the book its special biological significance is that he discusses the life history of these tailless batrachians, and their anatomy and physiology, with a variety of other forms that are either closely affined to them or more or less remotely connected. In summing up, Prof. Mivart shows the differences existing between a frog and a fish, a frog and a reptile, a frog and a bird, a mammal, and so on; and indeed what a frog really is, and he claims it to be "a tailless, lung-breathing, branchiate vertebrate, with four limbs typically differentiated, undergoing a complete metamorphosis, and provided with teeth along the margins of the upper jaw." This last character is one that distinguishes the frogs from the toads, while from other batrachians the frogs are at once separated by the absence of a tail.

In the United States we have at least fifty or sixty different species and subspecies of frogs and tree frogs, belonging to a number of different genera. The typical genus is the genus Rana, and to it belongs our common bullfrog (Rana Cateshiana), it at