In this brief sketch of some broad distinctions among the masses of animals we have a hint of the basis of their classification.
ANIMAL LIFE IN ORDERLY ARRANGEMENT
Classification is really only a sorting out of things into groups of the same kind. It may be artificial, according to fancy or convenience, or it may be by discovery of nature's inevitable development. It has been done crudely ever since men began to show curiosity about the things around them. They spoke of animals of the land, of the water, and of the air; of those that lived on vegetable fare as different from the flesh eaters; and in a more particular way they recognized various obviously like and unlike groups within the larger ones. All these distinctions were made on external appearance or behavior, and closer observation presently showed bad combinations, such as placing bats with birds simply because both flew, or whales with fish because both lived in water. Slowly it became evident that the only proper way to classify animals was by putting together those of like structure, and this could be accomplished only by intense comparative study of the interior anatomy of their bodies. Even here, however, progress was limited until the great light from the idea of organic evolution fell on biological science, by which it was perceived that the true criterion by which the proper place of any animal could be determined was its line of descent—a matter wherein the student of fossils could render, and has rendered, vast assistance. In other words a real, natural classification is according to ancestry, just as human relatives are grouped into families according to their known descent from the same forefather.
In this evolutionary light zoölogists have now perfected, at least in respect to its larger divisions, a classification of the animal kingdom which is generally accepted, and is followed in this book. It proceeds, reading downward, from the simpler and older forms of animal life to the more complex and more recent forms.
As to the names and relative order, or rank, of the subdivisions that we shall have occasion to mention, a few words are desirable. The only real fact is the individual animal. A collection of these so similar that they cannot be divided, and which will interbreed, but usually are sterile as to other animals, is termed a species. A number of species closely similar are bracketed together as a genus (plural genera), and this done, every individual is given a double name, as Felis leo to the lion, the first part of which indicates its genus, and is called its "generic" name, and the second indicates its species, and is called its "specific" name. This "scientific name" is given in Latin (or Latinized Greek) so that it may be unmistakably understood in all parts of the world, for a local name in one language would mean nothing to a student speaking some other language, or perhaps speaking the same language in another country; thus the name "robin" is applied to half a dozen very different birds in separate parts of the English-speaking world, and endless confusion would result were not each animal labeled in a language understood by everybody; and this must be a dead language, so that the significance of the terms applied shall not vary in place or time.
Several similar genera may form a family; families that agree in essential characteristics are united as orders; orders are grouped into classes; and finally like classes are assembled into a phylum (Greek, "a leaf": plural phyla), which is the largest division except the primary distinction of Protozoa and Metazoa.
[CHAPTER IV]
THE HUMBLEST OF ANIMALS—SPONGES
At the foot of the arrangement of phyla in the metazoa stand the Porifera, or sponges, fixed, plantlike, queerly shaped beings living in the sea, except one family in fresh waters, and abundant in all the warmer parts of the world on rocky bottoms. Whatever its size or shape, a live sponge (of which the commercial article is the more or less perfect skeleton) is coated with a thin fleshy membrane perforated by minute "inhalant pores" and larger holes termed "oscula," or mouths. Through the inhalant pores the sea water, with its burden of microscopic food, enters one of many spaces beneath the surface from which incurrent canals penetrate the interior of the sponge, constantly branching and growing smaller until lost to sight. The fine tips communicate with small cavities lined with cells that are fitted to seize and assimilate the nourishment brought them by the water. From these rudimentary stomachs go similar excurrent ducts that unite near the surface into trunk canals that carry out the used water and waste products. This system of circulation, bringing nutrient water strained through the pores, and expelling it forcibly after it has been cleared of food value, is kept in motion, with occasional periods of rest, by the action of "flagellate cells" that line certain tracts in the canals. These are elongated cells from which project whiplashlike filaments, one to each cell, whose movements in concert "resemble those which a very supple fishing rod is made to undergo in the act of casting a long line"—the movement being much swifter from without inward.