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.
Beneath the outer skin, and all among the canals and cavities, is a filling of gelatinous materials, largely protoplasm, in which are formed great numbers of variously branched and strengthening spicules, of limy material in one group, and in others of a flinty or glassy nature, or in the absence of these, a network of "spongin," such as forms the skeleton of our common washing sponges. Spongin is a substance allied to silk in chemical composition, and the threads are felted together in such a way as to form a firm, yet elastic structure. "In some Noncalcarea, which are devoid of spicules, the place of these is taken by foreign bodies—shells of Radiolaria, grains of sand, or spicules from other sponges. In others again, such as the Venus's flower basket (Euplectella), the glass-rope sponge (Hyalonema), and others, the skeleton consists throughout of siliceous spicules bound together by a siliceous cement."
Sponges are reproduced both by budding in some form, which is an asexual way, and by the sexual method of eggs and male cells; these are formed in the same sponge, but rarely at the same time, and the early stages of development are passed in a brood-cell within the body of the parent sponge. Finally, the embryo escapes through one of the outgoing canals, swims about awhile, becomes thimble-shaped, and settling down, fastens itself by the closed end to some patch of mud, a rock, dead shell or seaweed, closes the open end of the "thimble," and proceeds to grow.
Sponges do not appear to be eaten by fishes or anything else. Countless lower animals, such as marine worms, mollusks, and so forth, burrow into them, however, in search of shelter; and in reversal, certain small sponges, such as the cliona of our shores, burrow into the shells of mollusks, which explains the honeycombed appearance of many of the shells picked up on the beach. Sponges have a large part in that very interesting and widespread phase of marine life called "commensalism," in which two animals become intimately associated in a mutually beneficial way, and are thus spoken of as messmates. Some kinds of sponge are never found growing except on the backs or legs of certain crabs; the sponge conceals and protects the crab, while itself benefits by being carried from place to place, with constantly new changes of fresh water and food. This sort of partnership occurs in many different groups of marine animals.
The capture and preparation of sponges for market employ thousands of men and boats in the eastern Mediterranean, whence the best are derived, and in the West Indies and Gulf of Mexico, where the sponges are of a coarser kind, and are gathered and prepared by rougher methods. They are taken commercially also in other seas, and frequently dredged from vast depths.