elephant, and the comical shovels of the mole and ornithorhynchus, notwithstanding the great differences in their external appearance and use, contain essentially the same bones and the same arrangement of the bones as do the arms of men and women. The human body has two primary cavities in it. So have the bodies of all vertebrates: a neural cavity containing the brain and spinal cord, and a visceral cavity containing the heart, liver, lungs, and alimentary canal. Invertebrates have only one body cavity—the one corresponding to the visceral cavity of vertebrates—and the main nerve trunk, instead of extending along the back, as among vertebrates, is in invertebrates located ventrally. Vertebrates are the only animals on the earth that have a highly developed circulatory system, a system entirely shut off from the other systems, and containing a heart, arteries, veins, and capillaries. In all invertebrates the digestive and circulatory systems remain to a greater or less extent connected, the blood and food mingling more or less in the general cavity of the body. Worms and insects have pulsating tubes instead of heart and arteries. Crustaceans have hearts with one chamber, and mollusks have two or three chambered hearts, but the blood, instead of returning to the heart after its journey through the arteries, passes into the body cavity. In man and other vertebrates the circulating current is confined strictly to the bloodvessels, no particle of it ever escaping into the general body cavity. The heart of vertebrates is distinguished from that of invertebrates by being located ventrally. The heart of invertebrates is in the back. The blood of vertebrates differs from that of invertebrates in containing both red and white corpuscles. Invertebrates have white corpuscles only. Worms have yellow, red, or bright green blood. The blood of crustaceans is bluish, that of mollusks is white, and that of insects dusky or brown. The blood of all vertebrates, excepting amphioxus, is red. All backboned beings, whether they dwell in seas or cities, and whether they build nests or empires, have two eyes, two ears, nose and mouth, all located in the head, and always occupying the same relative position to each other. Invertebrates may have their brains in their abdomen, as do the mites; hear with their legs or antennae, as many insects do; see with their tunics, like the scallops; and breathe with their skin, as do the worms. The crayfish hears with its ‘feelers,’ the cricket and katydid with their fore-legs, the grasshopper with its abdomen, the clam with its ‘foot,’ and mysis and other low crustaceans have their auditory organs on their tails.