The parasites of man, and the diseases which proceed from them : a text-book for students and practitioners / by Rudolf Leuckart ; translated from the German, with the co-operation of the author, by William E. Hoyle.

Date:
1886
Catalogue details

Licence: Public Domain Mark

Credit: The parasites of man, and the diseases which proceed from them : a text-book for students and practitioners / by Rudolf Leuckart ; translated from the German, with the co-operation of the author, by William E. Hoyle. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

    37/808 (page 5)
    EFFECTS OF PARASITISM. their host; they must have organs of motion and of sense. This is in- variably found to be the case; temporary parasites possess powerful limbs (eg the bed-bug), sometimes even wings (midges and some other flies1), or'swimming appendages (fish-louse). When present, these organs aUow of a more complex development of the vital activities, and that, perhaps, to such a degree, that temporary parasites, when away from their host, display hardly any recognisable peculiarities. Only the nature of their food, and the way in which they obtain it, compel us to regard them as parasites; it is not the refuse of organic life, but living organisms, that supply them with nutriment. As the power of movement becomes less, it becomes more and more difficult for the parasite to leave its host. In this way a tem- porary gradually changes into a stationary parasitism ; the host which was formerly only visited at intervals, and for a short time, now serves as a shelter to the parasite, and is seldom abandoned by it or changed for another. Among stationary parasites there are many (e.g., the flea) which retain the power of movement, and sometimes abandon one host for another in search of a safer dwelling-place or more abundant nutriment. These forms present many analogies to the temporary parasites, not merely as regards their mode of life, but in their structure, especially in regard to the development of locomotor organs. In the majority of cases, however, the power of movement is reduced in stationary parasites, sometimes entirely lost, so that the animal remains for months, or even years, attached to the same host. Instances of this may be found in the bladder-worms and the female Lernseadee, which live with their heads imbedded in the muscles of fish. Moreover, it is not only the organs of locomotion which become abortive in these cases. The sense organs, and especially the eyes, whose development is almost co-extensive with the variety and energy of the muscular activity, in like manner frequently degenerate. The graceful outline of the body and its segmentation commonly dis- appear, in adaptation to the present slight need of locomotion. In fact, a glance at the group of the so-called intestinal worms, which are all stationary parasites, shows clearly that the more sedentary the life of a parasite becomes, the simpler and more undifferentiated is the form of the body. Moreover, the simplification of the external structure of the body is no more a special peculiarity of stationary parasites than is the possession of wings and swimming feet a peculiarity of free-living animals. Among the latter we find numerous examples of a similar form of body, and especially among those creatures with limited capabilities of locomotion, which, in this respect, are somewhat 1 Hippobosca, Ornithomyla, &c.