INFUSORIA AS PARASITES. 
295 
to the structural and reproductive phenomena of the Infusorial 
organism might he cited, but those already given suffice for 
existing exigencies. 
Proceeding to an enumeration of those representatives of the 
Infusorial series which fall within the legitimate scope of this 
article, the term Parasite, as here employed, at once confronts 
us and demands a brief exposition of its significance. Put a 
few years since, this term comprehended every organism that 
was, under any form, found associated with, or attached to, 
some other specific type. Credit is due, however, to the Belgian 
biologists, Paul and Edouard Yan Beneden, for having pointed 
out that there is a very large assemblage of animals usually 
comprehended under this collective term of Parasites, that in 
no way live at the expense of the animal organisms, generally 
of higher rank, with which they are found associated, but are 
beholden to them only for free lodgings, providing their own 
larder, or, at the most, contenting themselves with the crumbs 
that fall from their comrades’ table. For animals which main- 
tain towards their selected host so purely amicable a relation- 
ship the title of ‘ Parasite,’ implying an organism that lives 
upon or at the expense of another, has no true significance, and, 
recognizing the desirability of the introduction of a title that 
should more precisely indicate this peculiar relationship, the 
authors quoted have bestowed upon them the distinctive one of 
‘Commensals.’ Familiar examples of ‘ Commensalism,’ as dis- 
tinct from ‘Parasitism’ in its true and restricted sense, are 
afforded by such forms as the well-known, so-called Parasitic 
Sea-anemones, Sagartia parasitica and Adamsia palliata , found 
attached to the shells of Grasteropodous Mollusca inhabited 
by Hermit crabs ; and with which last-named Arthropods they 
are reported to cultivate so intimate an acquaintance as to se- 
cure their own transfer at the hands of their host to the next 
convenient abode which its constantly increasing bulk obliges it 
to occupy. 
The extensive tribe of the Cirripedia, including the 
Barnacles and Acorn- shells, yield also an extensive series of 
Commensals ; certain of them, such as the genera Tubicinella 
and Coronula , are respectively only found attached to, or deeply 
immersed within, the epidermis of various Cetacea ; while an- 
other form, Pyrgoma , takes up its abode within the midst of the 
polyparies of reef-building corals, and is met with under no 
other conditions. These, and innumerable other species ordi- 
narily denominated parasitic types, in no way prey upon the 
vital or nutrient juices of their elected host, after the manner 
of true Parasites, but, at the outside, simply take ‘ pot-luck’ and 
a small share of the good things consumed by the latter. Quite 
as frequently indeed they cater for themselves independently, 
