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those most oddly metamorphosed types, Peltogaster and Sacculina. 
The hosts of species that are referable to the parasitic category 
from among the ranks of the insect world are too innumerable, 
and, in many instances, too familiar, to need quotation ; even 
the comparatively small class of the Arachnida furnishes its 
quota towards the augmentation of the present list, as instanced 
by such types as the Pentastomum , Demodex, and the abundant 
representatives of the Tick family, or Ixodidoe. 
The object of the writer of this article is not, however, to 
descant at length upon the attractions presented by the mem- 
bers of the above-named highly-organized Metazoic groups, 
but to direct attention to an assemblage of organisms that 
exhibit parallel ecto- and endo -parasitic life-habits, and in 
many instances, precisely analogous, adaptive modifications with 
reference to their respective modes of existence, but which 
hitherto, on account chiefly of their exceedingly minute size, 
have received but scant notice. All the representatives of the 
group in view are, in point of fact, microscopic organisms, 
belonging to that subdivision of the Protozoic sub-kingdom, 
known as the Infusoria, for whose correct comprehension, and 
for a knowledge indeed of the very fact of their existence, we 
are dependent upon the assistance and revelations of the com- 
pound microscope. 
Before entering upon a survey of the very considerable 
series of Infusorial forms that are associated with some one or 
other of the many varied phases of parasitic growth, it is 
desirable, perhaps, to briefly indicate the broader lines of 
demarcation by which this assemblage of organisms is to be dis- 
tinguished from all collateral zoological groups, and also the 
more important secondary subdivisions, or orders, into which 
the Infusoria as a whole may be most naturally and conveniently 
subdivided. The Infusoria, as now most generally conceded, are 
to be regarded as unicellular animals, possessing individually the 
morphological value only of a simple histologic cell, having, 
in the majority of instances, a distinct cell-wall and en- 
closed nucleus, and multiplying abundantly by a simple pro- 
cess of binary subdivision. As here recognized, all infusorial 
forms possess, in addition to the foregoing essential cha- 
racteristics, locomotive appendages which take the form of cilia, 
flagella, or prehensile, and mostly suctorial, tentacles. With 
but rare exceptions, the Infusoria manifest the capacity of in- 
gesting solid food- substances, either by a distinct mouth, by many 
mouths, or through the general surface of the body; even yet 
more invariably, they possess one or more definitely located, 
rhythmically expanding and contracting spaces, serving as 
excretory organs, and to which the title of contractile vesicles 
is most usually applied. Various other phenomena pertaining 
