EFFECTS OF PARASITISM. 5 



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 (e.g., the bed-bug), sometimes even wings (midges and some other 

 flies 1 ), or swimming appendages (fish-louse). When present, these 

 organs allow 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 

 Lernseadse, 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, Ornithomyia, &c. 



