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remaining genera of the Opalinidse, Discophrya and Hoplitophryct , 
yield yet additional structural modifications, suggestive of the 
affinities last cited. In the first of these, an adhesive organ, 
analogous to that exhibited by numbers of Cestoid and other 
endoparasitic worms, taking the form of a sucker or acetabulum, 
is developed at the anterior extremity ; while in Hoplitophrya , a 
more or less complex series of horny hooklets is developed in the 
same region. These hooklets, as represented in Hoplitophrya 
armata (PI. VII. fig. 26), a parasite of the Earthworm, at once 
recall to mind the structures having a like form and consistence 
common to a very considerable number of parasitic worms, and 
which are utilized by them in a corresponding manner for 
securing a firm hold upon the intestinal wall of their chosen host. 
The animalcules referable to the Ciliate order of the Infusoria 
next to be mentioned, are distinguished for their ectoparasitic 
habits. Among these, the genus Gonchophthirus , represented by 
three known specific types, is found associated with the mucous 
hody-slime of various Mollusca. Gonchophthirus anodontce is 
thus, as its name implies, an ecto-parasite of the fresh- 
water Mussel ; while C. Steenstrupii (PL Till. figs. 8 and 9), 
is similarly attached to the Grar den- Snail, Helix hortensis. 
A yet more remarkable, and in this instance, highly instructive 
infusorial form, is encountered in the type described by 
M. Fouquet, under the title of Icthyophthirius multifilis. This 
species (PI. VIII. figs. 1-5) attaches itself to young trout, 
excavating more or less extensive depressions in the cuti- 
cular surface, and there multiplying to such an extent as to 
induce a morbid condition, and, unless checked in time, 
fatal results. The fish-hatching establishment in France 
recently suffered very severely through the attacks of this 
microscopic parasite. A remarkable fact is connected with 
the reproductive phenomena of this species ; it is one of 
the very few ciliate Infusoria that are propagated by the sub- 
division of its mass into a considerable number of minute spore- 
like elements. While rare among the Ciliate class of the 
Infusoria, the sporuloid mode of reproduction is of common, 
and almost universal, occurrence among the comparatively lowly 
organized Flagellata. 
Arriving at the order of Heterotricha, distinguished by 
their possession of a conspicuous series of larger adoral cilia, 
in addition to the finer ones which clothe the entire surface, 
the three genera, Balantidium , Nyctotherus , and Plagiotoma, 
are specially notable for their parasitic habits. Three out of 
five of the known species of the first-named genus occur in com- 
pany with the Opalinidae as endoparasites of the frogs and 
toads. One species, however, Balantidium coli (PI. VIII. fig. 10), 
has been obtained from the colon of the human subject ; while 
