The World Encyclopedia of Animals [antikvár]

A bird's-eye view ofthe animal kingdomThe classification of animalsOne and a half million species of walking, jumping, crawling, flying, burrowing,or swimming animals inhabit the earth. The study of the structure and behaviorof all these forms of animal life and the determination of their mutual relation-ships is the domain of zoology, the scientific study of animals. In such a study, itis necessary to order the great diversity of material as efficiently as possible andto bring all the phenomena within the confines of a system in which every speciesis assigned a logical and reasonable place.An ideal system would be one that ordered all the animals in accordance withtheir relationships, taking into account not only their external and internal struc-tures but also their manner of life, their development, and their evolution. Butthere are so many threads to be tracedevident, probable, and possiblethat it is impossible to construct a system in which they all play their proper role.One could approximate the natural relationships, to some extent, only by a com-plicated spatial model; and such a three-dimensional system would no doubt beextremely difficult to use. But if one mixes the natural elements with a (ratherarbitrary) quantity of artificial devices, one obtains a system which may well besomewhat simplistic, but which does at least have the advantage of practicality.(For an example, see illustration on page 11.)In this way one arrives at an initial division of the animal kingdom into a numberof large groups called phyla (singular, phylum), each of which is distinguished byone or more striking characteristics. There are many differences between an ele-phant, a canary, a viper, a salamander, and a goldfish; yet all these animals sharea number of important characteristics: they all have a skeleton composed of boneand cartilage, and a hollow, tubular central nervous system contained in the brainand the spinal cord. At some stage in their development they all possess a re-markable rod of cells enclosed by a tough membrane. The support offered bythese cells makes the whole structure, the notochord, a rather firm "temporaryskeleton." The notochord is so typical of these animals that one includes allspecies which are equipped with it, either temporarily or permanently, in thephylum of the chordates (Chordata). In all "vertebrate" animals, the notochord isquickly replaced during development by the vertebrae; it remains only in a fewprimitive "jawless" fish. Invertebrates such as the lancelet and theTunicata(Uro-chorda) are also entitled, thanks to their notochord, to claim a modest place inthe lowest portion of the chordate phylum.Within the phylum one distinguishes a number of classes, which are in turn sub-divided into ever-smaller groupings as follows: orders, families, genera (plural ofgenus), species, and races (varieties). But this breakdown is not yet sufficient forthe system builders; they have furnished the spiderweb of the system with a num-ber of even finer connective meshes.The binomial nomenclatureOnce Aristotle had catalogued the animal world of his day, only a few attemptswere made to improve his system. The fundamental principles which had beenenunciated by a few scholars of genius at the dawn of science remained unassailedThe zebraflnch (p. 2) is a well-knownaviary bird. The orange spot on the cheekis characteristic of the male, which canthus be easily distinguished from thefemale.The galago, or bush baby (facing page),is related to the loris but has a longer tailand larger ears (which it folds up whengoing to sleep). The galago has keen sensesand is a skillful insect-catcher. It makes ascreeching noise that resembles the cryingof nurslings; hence its alternate name"bush baby."