Let us trace, in a general way, the development or formation of the tail in the animal kingdom. One of the simplest and most generalized animals known, The raccoon’s tail am ringed all ’round; the ’possum's tail am bare; The rabbit’s got no tail at all; but the white spot’s always there. the minute, one-celled Amoeba proteus, is without constant body form. Therefore, there is no tail, just as there can be no head, no mouth, no eyes, no ears, and no legs—the mass of living substance or protoplasm constituting the species serving as all of these and more. In many other simple one-celled forms, however, the body is constant in outline and, in addition, in many of them one part of the body is always behind and another is always in front as the organism moves about. Here we find longitudinal orientation of the protoplasmic mass which constitutes the body and hence, as regards the tail, the foreshadowing of future developments.
The free-living, freshwater flatworm Planaria, a larger, more advanced, many-celled animal, offers a very perplexing problem when we attempt to divide it into general sections or parts according to the popular method. This curious organism is always cross-eyed, if its tiny "eye-spots" are really to be regarded as eyes, and its mouth is situated near the middle of the body. Now, if we consider the fore part of the animal as the head, including the mouth as usual, we find that most of the planarian is head, In fish, amphibians, reptiles, birds, and mammals, there is probably no part of the physiognomy that has been subjected to a more interesting series of variations and uses than this appendage. but if we regard the portion back from the "ears" or sensory lobes as tail, much of this head really proves to be something else. Thus, at this point we are still unable to positively separate head, body, and tail.
This same indefinite condition or status of the tail holds to a greater or lesser degree throughout the lower or invertebrate series of animal groups, although it is true that certain of these show caudal developments which are worthy of note. The crayfish has a series of five terminal plates which the animal often uses in swimming rapidly backward through the water. Numerous insects, such as the cricket, have conspicuous posterior hairs or filaments which some may regard as a tail. Bees have developed a strong sting behind. Scorpions, which also sting with an apparatus at the terminal part of the abdomen, likewise show no real tail—and so on.
In the back-boned animals or vertebrates the tail appears as a definite structure. Here, in the fish, amphibians, reptiles, birds, and mammals, there is probably no part of the physiognomy that has been subjected to a more interesting series of variations and uses than this appendage.
Take for instance, among the reptiles, a fossil form, the ancient Ichthyosaurus or "fish-lizard," was obviously specialized for pelagic life like the porpoise and the whale, but the most primitive modern species, the tuatara of New Zealand (Sphenodon punctatum) shows none of this. Its tail is merely strongly compressed along its length, as are those of many other reptiles which live in marshy or semiaquatic habitats—such as the crocodiles and their allies, or certain large South American lizards of the family Teiidae (Dracaena guianensis and Crocodilurus lacertinus). Such tails are long and strong, being obviously adapted to aid in swimming movements and in defense. A similar type of modification is found in the marine snakes which occur so commonly in the vicinity of the islands of the Pacific and Indian oceans. These creatures usually have the elongate body and other general characters of the serpents, but their tails, instead of being rounded as in the terrestrial species, are often much compressed, so as to form a helpful rudder which may be waved from side to side while the animal swims about.
A small lizard, the common spiny-swift of central Kansas, reminds one of a pleased dog as it quivers its little tail in anticipation of food.