16 OCCURRENCE OF PARASITES. 



after what has just been said, since these parts of the body, being on 

 the outside, are precisely the situations most convenient to parasites 

 of this kind. Where respiration is impossible, there can evidently be 

 no air-breathing parasites, and all notices of fly-larvae discovered in 

 such situations, as for example, within the internal urinary passages, 

 are to be regarded as mere fables. The absolute need of access of air, 

 which parasites of this description have, can be easily proved by 

 experiment. I have frequently introduced the larvae of Musca vomitoria 

 at all stages, even as eggs, into the body-cavity of dogs and rabbits 

 through apertures in the abdomen, but never in a single instance 

 observed any further development take place; in most cases they 

 died very shortly. 



From the foregoing remarks, it follows that parasites may be 

 divided into two groups ectoparasites (Epizoa, external parasites) 

 and entoparasites (Entozoa, internal parasites). I am well aware that 

 in certain cases this distinction is not more easy to make than that be- 

 tween internal and external organs, and that the two groups by no 

 means include all the peculiar forms of parasitic life ; but it is on the 

 whole convenient to retain it, to express the general conditions of 

 parasitism with which we are for the present concerned. 



The ectoparasite inhabits the most readily accessible organs of the 

 body of its host, which it frequently abandons at pleasure. The group 

 which we have already alluded to as temporary parasites are, with a 

 few exceptions, ectoparasitic. In the same way, the semi-stationary 

 parasites are usually found upon the outer skin, where the least 

 hindrance is offered to their movements, while the entirely stationary 

 parasites are more commonly met with in the internal organs. It 

 follows, therefore, that ectoparasites can generally be recognised as 

 such by their outward form, especially by the structure of their 

 organs of locomotion. 



In certain ectoparasites, which have but a slight locomotive capa- 

 city, there are usually found, either upon the organs of locomotion 

 (Fig. 2), or (as in ectoparasitic worms) in their stead, powerful organs 

 of attachment, which are generally more strongly developed than in 

 the Entozoa. These structures enable them to cling very firmly, and 

 prevent them from being detached by the movements of the animal 

 upon which they live. The great differences that exist between these 

 organs in different parasites are greatly dependent upon the mode of 

 life of their host and the structure of its outer skin. 



With regard to respiration, the ectoparasite, as has been already 

 remarked, depends upon its host, and shares with it the same condi- 

 tions of life. It usually possesses special organs of respiration, especially 

 when living upon terrestrial animals, and being, therefore, in direct 



