Imagens das páginas

the simplest fishes or fishlike forms. That is, the sea squirt begins life as a primitively simple vertebrate. It possesses in its larval stage a notochord, the delicate structure which precedes the formation of a backbone, extending along the upper part of the body, below the spinal cord. It is found in all young vertebrates, and is characteristic of the branch.

The other organs of the young tunicate are all of vertebral type. But the young sea squirt passes a period of active and free life as a little fish, after which it settles down and attaches itself to a stone or shell or wooden pier by means of suckers, and remains for the rest of its life fixed. Instead of going on and developing into a fishlike creature, it loses its notochord, its special sense organs, and other organs; it loses its complexity and high organization, and becomes a “mere rooted bag with a double

neck," a thoroughly deFig. 224.-The sea squirt or tunicate. generate animal.

A barnacle is another example of degeneration through quiescence. The barnacles are crustaceans related most nearly to the crabs and shrimps. The young barnacle just from the egg (Fig. 225, f) is a sixlegged, free-swimming nauplius, much like a young prawn or crab, with single eye. In its next larval stage it has six pairs of swimming feet, two compound eyes, and two large antenna or feelers, and still lives an independent, free-swimming life. When it makes its final change to the adult condition, it attaches itself to some stone or shell, or pile or ship's bottom, loses its compound eyes and feelers, develops a protecting shell, and gives up all power of locomotion. Its swimming feet become changed into grasping organs, and it


loses most of its outward resemblances to the other members of its class (Fig. 225, e). Certain insects live sedentary or fixed lives. All the members of the family of scale insects (Coccidae), in one sex at least, show degeneration that has been caused by quiescence. One of these coccids, called the red orange scale, is very

Fig. 225.-Three crustaceans and their larvae: a, Prawn, Peneus; b. Peneus, larva: c, Sacrulina, parasite; d. larva Sacculina; e, barnacle, Lepas, quiescent; f, larva of barnacle. (After Haeckel.)

abundant in Florida and California and in other orange-growing regions. The male is a beautiful, tiny, two-winged midge, but the female is a wingless, footless little sac without eyes or other organs of special sense, and lies motionless under a flat, thin, circular, reddish scale composed of wax and two or three cast skins of the insect itself. The insect has a long, slender, flexible, sucking beak, which is thrust into the leaf or stem or fruit of the orange on which the “scale bug” lives and through which the insect sucks the orange sap, which is its only food. It lays eggs or gives birth to young under its body, under the protecting wax scale, and dies. From the eggs hatch active little larval scale bugs with eyes and feelers and six legs. They crawl from under the wax scale and roam about over the orange tree. Finally, they settle down, thrust their sucking beak into the plant tissues, and cast their skin. The females lose at this molt their legs and eyes and feelers. Each becomes a mere motionless sac capable only of sucking up sap and of laying eggs. The young males, however, lose their sucking beak and can no longer take food, but they gain a pair of wings and an additional pair of eyes. They fly about and fertilize the saclike females, which then molt again and secrete the thin wax scale over them. Throughout the animal kingdom loss of the need of movement is followed by the loss of the power to move, and of all structures related to it. Loss of certain organs may occur through other causes than parasitism and Fig. 226.-The black scale, Lecanium olor. - a fixed life. Many insects and its parasite, the tiny chalcid fly, Scu- - - . tellista cyanea; and the ladybird beetle, live but a short time in Rhizobius ventralis. (After Isaacs.) their adult stage. May flies live for but a few hours or, at most, a few days. They do not need to take food to sustain life for so short a time, and so their mouth parts have become rudimentary and functionless or are entirely lost. This is true of some moths and numerous other specially shortlived insects. Among the social insects the workers of the termites and of the true ants are wingless, although they are born of winged parents, and are descendants of winged ancestors The modification of structure dependent upon the division of labor among the individuals of the community has taken the form, in the case of the workers, of a degeneration in the loss of


the wings. Insects that live in caves are mostly blind; they have lost the eyes, whose function could not be exercised in the darkness of the cave. Certain island-inhabiting insects have lost their wings, flight being attended with too much danger. The strong sea breezes may at any time carry a flying insect off the small island to sea. Probably only those which do not fly much survive, and so by natural selection wingless breeds or species are produced. Finally, we may mention the great modifications of structure, often resulting in the loss of certain organs, which take place to produce protective resemblances (see Chapter XIX). In such cases the body may be modified in color and shape so as to resemble some part of the environment, and thus the animal may be unperceived by its enemies. Many insects have lost their wings through this cause. When we say that a parasitic or quiescent mode of life leads to or causes degeneration, we have explained the stimulus or the ultimate reason for the degenerative changes, but we have not shown just how parasitism or quiescence actually produces these changes. Degeneration or the atrophy and disappearance of organs or parts of a body is often said to be due to disuse. That is, the disuse of a part is believed by many naturalists to be the sufficient cause for its gradual dwindling and final loss. That disuse can so affect parts of a body during the lifetime of an individual is true. A muscle unused becomes soft and flabby and small. Whether the effects of such disuse can be inherited, however, is open to serious doubt. Such inheritance must be assumed if disuse is to account for the gradual growing less and final disappearance of an organ in the course of many generations. Some naturalists believe that the results of such disuse can be inherited, but as yet such belief rests on no certain knowledge. If characters acquired during the lifetime of the individual are subject to inheritance, disuse alone may explain degeneration. If not, some other immediate cause, or some other cause along with disuse, must be found. We are accustomed, perhaps, to think of degeneration as necessarily implying a disadvantage in life. A degenerate animal is considered to be not the equal of a nondegenerate animal, and this would be true if both kinds of animals had to face the same conditions of life. The blind, footless, simple, degenerate animal could not cope with the active, keen-sighted, highly organized nondegenerate in free competition. But free

competition is exactly what the degenerate animal has nothing to do with. Certainly the Sacculina lives successfully; it is well adapted for its own peculiar kind of life. For the life of a scale insect, no better type of structure could be devised. A parasite enjoys certain obvious advantages in life, and even extreme degeneration is no drawback, but rather favors it in the advantageousness of its sheltered and easy life. As long as the host is successful in eluding its enemies and avoiding accident and injury, the parasite is safe. It needs to exercise no activity or vigilance of its own; its life is easy as long as its host lives. But the disadvantages of parasitism and degeneration are apparent also. The fate of the parasite is usually bound up with the fate of the host. When the enemy of the host crab prevails, the Sacculina goes down without a chance to struggle in its own defense. But far more important than the disadvantage in such particular or individual cases is the disadvantage of the fact that the parasite cannot adapt itself in any considerable degree to new conditions. It has become so specialized, so greatly modified and changed to adapt itself to the one set of conditions under which it now lives, it has gone so far in its giving up of organs and body parts, that if present conditions should change and new ones come to exist, the parasite could not adapt itself to them. The independent, active animal with all its organs and all its functions intact, holds itself, one may say, ready and able to adapt itself to any new conditions of life which may gradually come into existence. The parasite has risked everything for the sake of a sure and easy life under the presently existing conditions. Change of conditions means its extinction.

« AnteriorContinuar »