6. All bloodless animals, except a few marine species of the cephalopoda, are smaller than those which have blood. These animals are the largest in warm waters, and more so in the sea than on the land, and in fresh water. All creatures that are capable of motion are moved by four or more limbs. Those with blood have four limbs only, as man has two hands and two feet. Birds have two wings and two feet; quadrupeds and fishes have four feet or four fins. But those animals which have two wings or none at all, as the serpent, are nevertheless moved by four limbs; for the bendings of their body are four in number, or two when they have two wings. 7. Those bloodless animals which have more than four feet, whether furnished with feet or wings, always have more than four organs of locomotion, as the ephemera, which has four feet and four wings; and in this it not only agrees with its peculiar manner of life, from which also it derives its name, but also that it is winged and four-footed; and all creatures, whether they have four feet or many feet, move in the same direction, for they all move in the long way of their bodies. All other animals have two leading feet, the crab alone has four. CHAPTER VI. 1. The following are the principal classes which include other animals--birds, fishes, cetacea. All these have red blood. There is another class of animals covered with a shell, and called shell fish, and an anonymous class of soft-shelled animals (malacostraca), which includes carabi, carcini, and astaci; and another of mollusca, such as teuthis, teuthos, and sepia; and another class of annulose animals. All these are without blood, and the species with feet have many feet. There are no large classes of other animals; for there are many forms which are not included under a single form, but either stand alone, having no specific difference, as man, or have specific differences, but the classes are anonymous. 2. All animals with four feet and no wings have blood. Some of these are viviparous, others oviparous. The viviparous are not all covered with hair, but the oviparous have scales. The scale of a reptile is similar in situation to the scale of a fish. The class of serpents, sanguineous land animals, is naturally without feet. Though some have feet, this class is also covered with scales. All serpents, except the viper, are oviparous. The viper alone is viviparous, so that not all viviparous animals have hair; for some fishes also are viviparous. All animals, however, that have hair are viviparous; for we may consider the prickles of the hedgehog and porcupine as analogous to the hair of animals; for they answer the purpose of hair, and not, as in marine animals that are so covered, of feet.[25] [25] The Echinidæ.