RESPIRATION OF PARASITES. 13 



in the case of Strongylus gigas, whick is far more abundant in car- 

 nivorous than in herbivorous animals, while it has only been met with 

 a few times in man and other hosts. By the help of the statistical 

 method it is easy to find out what are the animals most frequented by 

 a given parasite, and the results obtained show that the several hosts 

 are always more or less allied to the one in which the parasite is 

 most commonly found. The causes of this are no doubt various, and 

 partly of a kind which will be discussed later on, when we come to 

 examine into the life-histories, origin, and migrations of parasites ; 

 but for the present it may be remarked that these causes are to be 

 looked for partly in the hosts themselves (in their distribution, habits, 

 manner of locomotion, and their food), partly also in the nature, con- 

 dition, and needs of the parasites. 



The factors which we are considering now are nearly the same as 

 those which govern the relations between carnivorous and herbivorous 

 animals, inasmuch as they concern not merely the actual carnivorous 

 instinct, but also the choice of the prey. We need not be surprised, 

 therefore, that a carnivorous mode of life, as has been already pointed 

 out, is unmistakeably related to a parasitic mode of life. 



A very cursory examination of the conditions of life proves that 

 we are right in regarding the distribution of parasites as greatly de- 

 pendent upon the nature of the host as well as of the parasite itself 

 It is clear, for instance, that a parasite with lungs and other organs 

 that need a direct contact with the air, can only exist in those 

 creatures whose structure and mode of life render this possible, and 

 only in those parts of the body to which the air has free access. Thus, 

 all parasitic air-breathing insects (including Arachnida) are, without 

 exception, confined to terrestrial or amphibious animals, and generally 

 to their external skin. The walrus, for example, harbours a Pediculus 

 of considerable size. On the other hand, the external parasites of 

 aquatic animals generally belong to the Crustacea or some group 

 which breathes by means of gills, and therefore needs direct contact 

 with water. "Worms breathe by means of the skin, and hence the 

 parasitic members of this group — the so-called Helminths — are some- 

 times found as ectoparasites upon aquatic animals, while they are 

 usually met with only in the interior of terrestrial animals, in organs 

 where they are bathed in the oxygenating fluids of their host. Para- 

 sitic worms are also found in similar situations within the bodies of 

 aquatic animals, but this is quite intelligible, inasmuch as they are 

 of all parasites the most widely diffused ; in fact, internal parasites, or 

 " Entozoa," as they are usually termed, are mainly worms. 



With this wide distribution may be coupled the fact that parasitic 

 worms are numericalljiQfe'^M@f4^M9S?ftOffilh parasitic Arthropoda ; 



