Most animals appear first as eggs laid by the mother. This is true of the birds, the reptiles, the fishes, the insects, and most of the hosts of invertebrate animals. This egg may be cared for by the parent as with the birds, or simply deposited in a safe place as with most insects, or perhaps dropped without care into the water as with most marine invertebrates. The young animal which issues from the egg may at the time of its hatching resemble the parent in appearance and structural character (although always much smaller) as with the birds, some of the insects, and many of the other animals. Or it may issue in a so-called _larval_ condition, in which it resembles the parent but slightly or not at all, as is the case with the gill-bearing, legless, tailed tadpole of the frog or the crawling, wingless, wormlike caterpillar of the butterfly, or the maggot of the house-fly. =Life-history.=--Any animal which hatches from an egg has undergone a longer or shorter period of development within the egg-shell before hatching. The development of an animal from first germ-cell to the time it leaves the egg, for example, the development of the embryo chick from the first cell to time of hatching, is called its _embryonic_ development; and the development from then on, for example, that of the chick to adult hen or rooster, or that of tadpole to frog, is called the _post-embryonic_ development. Beginning students of animals cannot study the embryonic development (_embryology_) of animals readily, but they can in many cases easily follow the course of the post-embryonic development, and this study will always be interesting and valuable. When the "life-history" of an animal is spoken of in this book, or other elementary text-book of zoology, it is the history of the life of the animal from the time of its birth or hatching to and through adult condition that is meant, not the complete life-history from beginning single egg-cell to the end. In all of the study of the different kinds of animals to which the rest of this book is devoted, attention will be paid to their life-history. PART II SYSTEMATIC ZOOLOGY CHAPTER XIV THE CLASSIFICATION OF ANIMALS