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short excursions from the water. Still others (mud fishes) retain the primitive lunglike structure of the swim bladder, and are able to breathe air when, in the dry season, the water
of the pools is reduced to mud.
Another series of adaptations is concerned with the places chosen by animals for their homes. The fishes that live in the water
have special organs for Fig. 205.-Tree toad, Hyla regilla. breathing under water
(Fig. 204). Many of the South American monkeys have the tip of the tail adapted for clinging to limbs of trees or to the bodies of other monkeys of its own kind. The hooked claws of the bat hold on to rocks, the bricks of chimneys, or to the surface of hollow trees, where the bat sleeps through the day. The tree frogs or tree toads (Fig. 205) have the tips of the toes swollen, forming little pads by which they cling to the bark of trees.
Among other adaptations relating to special surroundings or conditions of life are the great cheek pouches of the pocket gophers, which carry off the soil dug up by the large shovellike feet when the gopher excavates its burrow.
Those insects which live underground, making burrows or tunnels in the soil, have their legs or other parts adapted for digging and burrowing. The mole cricket (Figs. 206 and 207) has its legs stout and short, with broad, shovellike feet. Some Fig. 206. — The mole water beetles and water bugs have one cricket, Gryllotalpa,
with fore legs modior more of the pairs of legs flattened and
fied for digging. broad to serve as oars or paddles for swimming. The grasshoppers or locusts, which leap, have their hind legs greatly enlarged and elongated, and provided with strong muscles so as to make of them “leaping legs." The
grubs or larvae of beetles which live as “borers” in tree trunks have mere rudiments of legs, or none at all. They have great, strong, biting jaws for cutting away the hard wood. They move simply by wriggling along in their burrows or tunnels. Insects that live in water either come up to the surface to breathe or take down air underneath their wings, or in some other way, or have gills for breathing the air which is mixed with the water. These gills are special adaptive structures which present a great variety of form and appearance. In the young of the May flies they are delicate platelike flaps projecting from the sides of the body. They are kept in constant motion, gently waving back and forth in the water so as to maintain currents to bring fresh water in contact with them. Young mosquitoes do not have gills, but come up to the surface to breathe. The larvae, or wrigglers, breathe through a special tube at the posterior tip of the body, while the pupae have a pair of hornlike tubes on the back of the head end of the body. Many fishes, chiefly of the deep seas, develop organs for producing light. These are known as luminous organs, phosphorescent organs, or photophores. These are independently developed in four entirely
o unrelated groups of fishes. This difference Fig. 207.-Front leg in origin is accompanied by corresponding of the mole cricket,
differences in structure. The best known showing at e opening of auditory or
type is found in the Iniomi, including the gan. (After Sharp.) lantern fishes and their many relatives.
They may have luminous spots, differentiated areas, round or oblong, which shine starlike in the dark. These are usually symmetrically placed on the sides of the body. They may have also luminous glands or diffuse areas which are luminous, but which do not show the specialized structure of the phosphorescent spots. These glands of similar nature to the spots are mostly on the head or tail. In one genus, AEthoprora, the luminous snout is compared to the headlight of an engine. Entirely different are the photophores in the midshipman or singing fish (Porichthys), a genus of the toad fishes or Batrachoididae. These species live near the shore and the luminous spots are outgrowths from pores of the lateral line.
In one of the anglers (Corynolophus reinhardt) the complex bait is said to be luminous, and luminous areas occur on the belly of a very small shark of the deep seas of Japan (Etmopterus lucifer). Dr. Peter Schmidt of St. Petersburg has a drawing of this shark made at night from its own light.
While among the higher or vertebrate animals, especially the fishes and reptiles, most remarkable cases of adaptations occur, yet the structural changes are for the most
Fig. 208.-Nest of the trapdoor spider. part external, usually not
the development of the internal organs other than the skeleton. The organization of these higher animals is much less plastic than that of the invertebrates. In general, the higher the type the more persistent and unchangeable are those structures not immediately exposed to the influence of the struggle for existence. It is thus the outside of an animal that tells where its ancestors have lived. The inside, suffering little change whatever the surroundings, tells the real nature of the animal.
PARASITISM AND DEGENERATION
Les causes de l'évolution regressive peuvent se ramener à une seule, la limitation des moyens de subsistance, de là, la lutte pour l'existence entre les organismes ou les societés, et entre leurs parties composantes.- DEMOOR, MASSART, and VANDERVELDE.
A SPECIAL kind of adaptation is that shown by parasitic animals. The relations of parasitic animals to their hosts appear in many familiar examples, and the results of this parasitic life, or at least the conditions that seem always to attend it, namely the degeneration, slight or extreme, of the parasites, is also familiar to all observers of animal life. The term parasitism, as well as the term degeneration, cannot be very rigidly defined. To prey upon the bodies of other animals is the common habit of many creatures. If the animals which live in this way are free, chasing or lying in wait for or snaring their prey, we speak of them in general as predatory animals. But if they attach themselves to the body of their prey or burrow into it, and are carried about by it, live on or in it, then we call them parasites. And the difference in habit between a lion and an intestinal worm is large enough and marked enough to make very clear to us what is meant when we speak of one as predatory and the other as a parasite. But how shall we class the lamprey, that swims about until it finds a fish to which it clings, while sucking away its blood? It lives mostly free, hunting its prey, clinging to it for a while, and is carried about by it. Closely related to the lampreys are the hag fishes (Jlyrine) marine eellike fishes that attach themselves by a suckerlike mouth to living fishes and gradually scrape and eat their way into the abdominal cavity of the host. These “hags” or “borers” approach more nearly to the condition of an internal parasite than any other vertebrate. And what about the flea? In its immature life it lives as a white grub or larva in the dust of cracks and crevices, of floors and cellars and heaps of débris; here it pupates, and finally changes into the active leaping blood-sucking adult which finds its way to the body of some mammal and clings there sucking blood. But it can jump off and hunt other prey; it leaves the host body entirely to lay its eggs, and yet it feeds as a parasite, at least it conforms to the definition of parasite in the essential fact of
Fig. 209.–At the left, the red-tailed trichina fly, Winthemia, l-pastulata, the parasite
of the army worm, Leucania unipuncta; at the right, the worms upon which the fly has laid eggs. (After Slingerland.)
being carried about on or in the host body, while feeding at the host's expense.
It is of course not particularly important that we distinguish sharply between parasitic and predaceous animals, but as we look on the degeneration of parasitic animals as the result of their special habit of life, we must attempt a sort of classification of the phases or degrees of parasitism, in order to associate with them corresponding categories of degeneration.
The bird lice (Mallophaga), which infest the bodies of all kinds of birds and are found especially abundant on domestic fowls, live upon the outside of the bodies of their hosts, feeding upon the feathers and dermal scales. They are examples of external parasites. Other examples are fleas and ticks, and the crustaceans called fish lice and whale lice, which are attached to marine animals. On the other hand, almost all animals are infested by certain parasitic worms which live in the alimentary canal, like the tapeworm, or imbedded in the muscles, like the trichina. These are examples of internal parasites. Such parasites belong mostly to the class of worms, and some