OBSERVATIONS ON ENTOZOA. 
897 
referring to the study of Helminths, Entozoa, and Parasites, 
are almost synonymous. Thus, all of them embrace a know¬ 
ledge of the facts relating to creatures which, at some time 
or other in the course of their lives, reside in the bodies of 
other creatures, and whose presence during the period of that 
residence, be it ever so long or brief, is liable to prove incon¬ 
venient or injurious to the animal attacked. Certainly, of 
the three, the term Parasitology is the most comprehensive, 
inasmuch as it embraces all the external as well as the internal 
parasites. 
I may here remark that helminthologists frequently employ 
the term host to designate the bearer of the parasite; this 
distinctive term being equally applicable in the case of man¬ 
kind as well as in animals. In like manner, we employ 
the term guest for the parasite, as also convenient and ex¬ 
pressive of the peculiar relation subsisting ‘between the 
invader and the invaded. In the case of those “ hosts” which 
merely entertain the guests, so to speak, as fellow-boarders, 
the latter may be appropriately spoken of as “ welcome 
guests and, in the case of the true Entozoa, which re¬ 
morselessly extract nutriment from the tissues of the invaded 
animal, the expression tf< unwelcome guests” becomes equally 
distinctive and applicable. 
In whatever way you view this important subject of Para¬ 
sitism—even, you see, whilst merely explaining terms a know¬ 
ledge of which is essential to a perfect comprehension of the 
study—the strangest reflections inevitably crop up, one by 
one, before the mind’s eye. Of these, those which have 
reference to the explanation of the possible mode or modes of 
origination of these singular creatures are the most difficult 
to deal with and to shake off. 
Note, particularly, that the parasites either comprise or, in 
some cases, form small parts of certain more or less well- 
known natural groups of animals; therefore, whatever spe¬ 
cial purposes you may suppose to have been fulfilled by the 
creation of parasitical animals, it is quite clear that you 
cannot disconnect them from the intimate zoological re¬ 
lationships which they hold with the non-parasitical 
creatures. The full value and significance of this statement 
is, I dare say, not yet apparent to you; therefore, to put the 
argument in a simpler form, what I mean is this:—In the 
course of your anatomical inquiries you may be struck with 
astonishment at the apparent evidences of design manifested 
in the construction and adaptations observable in the organi¬ 
sation, let us say, of the horse. Very well. If this be so, 
let me tell you beforehand that we shall find the same 
