INFUSORIA AS PARASITES. 
303 
interesting circumstance in connection with, the life-history of the 
multinucleate species of the genus Opalina , here referred to, that 
they commence existence with the possession of only a single 
endoplast or nucleus, and, after arriving at the full size or multi- 
nucleate condition, become subdivided by repeated fission into 
smaller and smaller fragments that possess finally only a single 
endoplastic element, each such ultimate fragment, after encyst- 
ment and a more or less prolonged quiescent state, repeating 
the developmental cycle. The several species of Opalina proper 
occur invariably as endoparasites of the intestinal viscera of the 
tailed and tailless Amphibia ; the most familiar type, as illus- 
trated by Opalina ranarum (Pl. VII. figs. 21-25), being an almost 
constant guest of the common Frog (Rana temporaria). 
The second generic group of the Opalinidae, upon which 
Prof. Stein has conferred the title of Anoplophrya, — occurring 
abundantly as endoparasites of marine and fresh-water worms, 
molluscs, and other invertebrata, — exhibits a marked advance in 
complexity upon the one last described, there being in addition 
to a conspicuously developed endoplast, which mostly takes a 
band-like form, and is produced centrally through the entire 
length of the body, an equally distinct contractile vesicular 
system. Such system, moreover, is not represented, as in the 
majority of the Infusoria, by a single rhythmically contractile 
space, but by a number of such spaces, which are generally dis- 
tributed in a single, or it may be a double, chain-like series, down 
one or both of the lateral borders. It is this monadiform modi- 
fication of the disposition of the structures in question that 
doubtless leads to that lineal and canal-like development of this 
structure that is met with in certain other types, and which, 
indeed, by artificial pressure, may be produced in the forms 
now under consideration. A highly interesting circumstance, 
pertaining to the development phenomena of certain representa- 
tives of the genus Anoplophrya , is connected with the peculiar 
modification of the ordinary process of binary subdivision. In 
most instances, it happens that the animalcule so subdividing, 
becomes separated, either transversely or longitudinally, into 
two equal or sub- equal moieties. Here, however, it more often 
happens that a small and altogether unequal fragment becomes 
separated from the posterior extremity ; while in occasional 
instances, such as Anoplophrya prolifer a (PI. VII. fig. 27), an 
endoparasite of a marine Planarian, a number of small frag- 
mentary portions are developed simultaneously in the posterior 
region of the same zooid, and become separated off in consecu- 
tive order. As already recognized by various biologists, this 
reproductive phenomenon accords in a most remarkable manner 
with what obtains among the Tseniadae and other Cestoid worms, 
in their so-called ‘ proglottid 9 reproductive phase. The two 
