Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/281

ANIMAL.] PARASITISM 263 specialized function similarly reduced to a trace. Along with this, or rather as a corollary of it, conies the concep tion of economy of unused structure ; our notions of specialization become henceforth associated with a corre sponding possibility of simplification, and our idea of pro gress must be for ever accompanied by the corresponding possibility of degeneration. The conditions of parasitic life are readily seen to differ primarily from those of independent organisms in negative characters, i.e., in the simplification of the factors of the environment ; let us therefore .briefly consider the results of such progressive simplification upon organisms in general. Let the mechanical conditions be simplified by the cessation of active movement ; the specialized body -form necessary for locomotion then becomes unnecessary ; locomotor muscles and their skeletal attachments are simplified or dis appear ; organs of sense are far less needed ; and nervous adaptations and structures become correspondingly reduced. In all these respects then sessile parasites simply agree with other sessile animals. Again, let us simplify the environment by the deprivation of light ; eyes and pigment are useless, and our organism, whether cave-dwelling insect or crustacean or internal parasite, becomes blanched and blind; and similarly with other senses. Or let us subtract as far as possible the element of danger from other animals by special protection or concealment in one of the " nooks of life " ; here again for shelled mollusc, sand buried AmphioxuS) or hidden parasite the diminished need of nervous adaptations is a similar degenerative factor. Let food become abundant, the same nervous economy follows; let it be highly nutritive, and digestive structures and func tions may be simplified ; thus the examples of progressive degeneration of the alimentary system up to its complete replacement by superficial absorption, afforded by various parasitic series, are natural enough. The soft integument unprotected and blanched, the reduced muscular activities, the simple or absent alimentary tube, the reduced circu latory and respiratory organs consequent upon diminished waste and softened integument, are all intelligible enough, as also is the increase in reproductive activity demanded by increased risk of failure to find the appropriate condi tions. The few adaptive conditions are readily understood : given the continuous application of a flat muscular surface to resist detachment from the host, and atmospheric pres sure helps the development of the sucker; given either a clutching limb or a portion of the body-wall thrust for support into the host, and the mechanical conditions aid the differentiation of a hook ; here, if anywhere, function in fact may be said to make the organ, and such curious resemblances of superficial form as those between say a gregarine, a tapeworm, and an Echinorhynckus are not hard to explain. Further details of the process of retrograde metamor phosis and of the enormously important phenomena of degeneration cannot here be attempted ; it must suffice if the general dependence of such changes upon simplification of environment freedom from danger, abundant alimen tation and complete repose, &c. (in short, the conditions commonly considered those of complete material well- being) has been rendered clearer, and if the phenomena of parasitism, however apparently aberrant, become intel ligible as new evidences of the unity of organic nature. 1 Effects of Parasite on Host. As the result of the associa tion of two organisms with more or less constancy, various mutual modifications of form and function must obviously occur. The more important effects of parasite on host may be briefly outlined. Semper cites numerous cases where the commensal or parasite has a mechanically trans- i See Dolirn, D. Princip. d. Functioiisicechsel; Lankester, On Degeneration, London, 1880; Semper, Animal Life. forming effect on the host. Thus a horny coral with which an annelid is constantly associated has become permanently modified to form an encasing tube. Worms inside corals have enlarged the base of the cavity by stimulating growth, and may also produce permanent pores. Pycnogonids on Campanularia produce galls, which ac quire specific characters, and various species of crab para sitic on corals form galls, two of which coalescing, form a sort of " cave dwelling " with two fissures which are kept open by the respiratory currents of the crab, which thus both stimulates and checks the growth of the polyps. In higher animals, and with more intimate parasitism, the mechanical influences of the parasite on the host are more serious and more markedly pathological. Thus parasitic worms, by their size and number, frequently close up passages such as arteries, windpipe, ic., causing often fatal results. But many parasites are also actively de structive to certain tissues of their host thus, as Semper points out, Pdtoyaster destroys the female reproductive organs of Payurus, a Trematode those of Limnxa stagnalis, the larva of a fly (Cuterelra emasculator) the testes of various species of American squirrel. In none of these cases, how ever, is the general vitality of the host affected. The results of active motion within the host are productive of still more serious mischief; thus the internal migration and burrowing of such parasites as Trichina and Bilharzia, is well known to produce violent inflammation. The per foration of vessels, the consequent extravasation of blood, and the destruction of tissue often end fatally for the host. Leuckart distinguishes pathological effects as due either to growth and increase of parasites, or to their wanderings within the host, or thirdly to the very consider able loss of nourishment which a number of parasites of appreciable size necessarily entails. Some blood-sucking parasites are specially dangerous, and many less ferocious forms doubtless poison their host to some extent by their waste products. Roux also notes how parasites an Echinococcus, for example by inducing a flow of nutritive material, may develop a net work of capillaries and produce other histological changes 2 It is probable that many of the most remarkable integumentary specializations of the animal kingdom are defences against parasites (somewhat as the stings or thorns which protect foliage, or the hairs which keep ants from flowers) ; thus the nematocysts of coelenterates, the mol- luscan shell or the crustacean mail, the vigilant pedicellariaj of the echinoderm, or the scales of the fish are alike largely specialized as defences against the never-ceasing attacks of swarms of larval parasites, eagerly struggling to gain entrance or footing anywhere. The history of the medical aspects of parasitism can only be very briefly alluded to. From the time of the ancient Arabian physicians some diseases, such as itch, have been referred to parasites. With the increasing knowledge as to the prevalence and importance of parasitism there arose a distinct parasitic theory of disease, and in the 17th and 18th centuries such questions were discussed as "an mors naturalis sit substantia verminosa." In spite of the gradual unravelling of the mysteries of origin and life- history, physicians long clung conservatively to the old hypothesis of spontaneous generation, even Bremser regarding the pathological states of the host not as caused by the parasites, but as causing and in fact creating them. It was not till within the last thirty years that, with the rise of experimental helniinthology, medical science shook itself free from superstition and ignorance, and devoted close attention to aetiology and treatment, culminating in that systematic warfare against all forms of parasitism 2 Leuckart, op. cit. Semper, Animal Life; Roux, D. Kampf d. Theile im Organismtts ; Ziegler s Pathology, &o.