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but exhibiting a closer relationship to the Scolecida and other 
mouthless endoparasitic worms. As now more familiarly known 
there can, however, be but little doubt that these animalcules 
must be regarded as specially modified and retrograde repre- 
sentatives of the ordinary Holotrichous Ciliata, which, by long 
exposure to an endoparasitic mode of existence, have in a 
parallel manner lost all trace of an oral system, and developed 
a capacity of absorbing the nutritious and protein-laden fluids 
in which they are constantly immersed, through the general 
surface of their cuticle. As originally instituted, the group of 
the Opalinidae included only the single representative genus 
Opalina ; recent research has, however, so extended the number 
of known species that it has been found convenient to separate 
them into four generic groups, each distinguishable by well- 
marked and highly characteristic structural modifications. In 
the simplest integer of this series, for which alone the original 
generic name of Opalina is now retained, structural differentia- 
tion is so rudimentary or obscure that its members were for a 
long while reported as being entirely devoid even of those two 
elementary histologic elements which, with scarcely an excep- 
tion, are distinctly recognizable in every other known member 
of the infusorial world, and represented by the endoplast, or 
nucleus, and rhythmically expanding and contracting spaces 
known as contractile vesicles. That these simple Opalince. are 
devoid of the structure last referred to has been confirmed by 
the latest and most exhaustive investigations ; the same re- 
searches, as conducted by Engelmann and Ernst Zeller, have, 
however, revealed the fact that endoplasts, or nuclei, are not 
only invariably represented, but in many species occur in each 
adult individual zooid as innumerable very minute spheroidal 
bodies distributed at short intervals throughout the entire sub- 
stance of the cortex, the animalcules in such cases, like some 
few other Infusoria and the Foraminifera, being essentially 
multinucleate. By some, it is held that this multinucleate 
structure of the Opalince militates substantially against their 
acceptance as unicellular organisms, and indicates their 
closer affinity to the multicellular or metazoic animal series. 
Neither here, however, nor in any other of the multinucleate 
representatives of the sub-kingdom Protozoa, is the slightest 
trace exhibited of a tendency of the body protoplasm to become 
subdivided into subordinate or cellular areas, as is essential to 
the composition of the metazoic body. It is nevertheless evi- 
dent, that, in all these multinucleate forms a first step is made 
towards an acquirement of this higher metazoic organization, 
and that, it is in association with such abnormal types that the 
transition from one to the other of these two primary zoological 
groups is, if anywhere, to be successfully sought. It is an 
