64 General Part. 



boundary between the external and internal organs is not clearly 

 defined. Most parasites do not spend their whole lives as such, 

 but lead an independent existence at one time or another. Some 

 are, for example, • parasitic when young, but free-living as adults 

 (the Gadflies), whilst others, conversely, are first free-living and 

 later on become parasitic (parasitic Crustacea). This mode of life- 

 has a marked effect on the structure of the animal : it is often 

 relatively slight in the temporary parasites, and in those stationary 

 ectoparasites which can move about freely on the body of the 

 host; but very great in most stationary ecto- or endo-parasites. 

 The fact that nutriment is plentiful and immediately accessible 

 exercises its usual effect : the power of locomotion is decreased ; a 

 more or less complete reduction of the limbs is seen : the sense 

 organs, further, e.g., the eyes, are much affected : most stationary 

 forms, especially the endoparasites, are blind. Organs of adhesion, on 

 the other hand, are frequently well- developed, as suckers, hooks j 

 or certain limbs are modified for this purpose. Parasitism, moreover, 

 exerts a definite influence upon the whole life. A natural result is, 

 for instance, that the animal must generally undertake migrations, 

 i.e., that it must not spend the whole of its life, from the ovum to 

 sexual maturity, in the same host, but must be actively or passively 

 transported at some period into another. Besides this one migration, 

 there are often others. Many parasites regularly produced as eggs 

 in one host, or in the free state, live for some time in another, the 

 intermediate host, and finally attain sexual maturity in a third. 



Parasites belong to various sections of the Animal Kingdom. There are, 

 nevertheless, certain large groups of which none or only a few individuals are 

 modified in tHs direction. Among the Vertebrata, only a single Pish : so also 

 only a few Molluscs, Ohsetopods, Ccelenterates, and absolutely no EcMnoderms 

 live in this way. The Arthropoda, especially the Crustacea, furnish a large 

 contingent, which are for the most part, ectoparasites, the Crustacea, exclu- 

 . sively upon aquatic animals, the others almost as exclusively upon land animals. 

 Further, amongst Annelids, a great many Leeches are ecto-parasites : most of the 

 Nemathelminths are endoparasitic : whilst of the numerous parasitic Platy- 

 helminths, some are endo- and some ecto-parasitic. The parasitic Round- and 

 Plat-worms are frequently called intestinal (Entozoa). Many Protozoa lead 

 this life, Gregarines, many Infusoria, etc. Many of the members of all the chief 

 divisions of animals must serve as hosts, especially the Vertebrata, which 

 principally on account of their large size, complicated structure, and relatively- 

 long life, afford an excellent scene of action for parasites, both internal and 

 external. Many of these creatures are limited as regards the choice of a host ; 

 some live, for instance, always in one species, never in any other; others are 

 confined to a few allied forms ; others, again, have a larger, but always a 

 restricted choice. The same parasite, for instance, cannot live indiscriminately 

 in a Pish and in a Mammal. On the other hand, it frequently happens that 

 the same form hves, at different periods of development, in animals of a very 

 different systematic position; Echinorhynchus, for example, is found, when 

 young, in Arthropods, as an adult, in Vertebrates. 



Some animals, e.g., some Leeches, form a transition from the pi'edatory to the 

 temporai-y parasitic condition, for sometimes they eat small animals, and 



