INFUSORIA AS PARASITES. 
305 
the fifth, form, B. medusarum , is described by Mereschkowsky as 
taking up its residence within the alimentary and radial canals 
of various small Medusae, Encope and Bougainvillia , obtained 
from the White Sea. Among the Heterotrichous genera just 
enumerated, the highest physiological interest is undoubtedly 
attached to that of Nyctotherus. Its several representatives 
inhabit, as endoparasites, the intestinal viscera of various 
insects and myriopods, or in the case of N. cordiformis 
of Frogs and Toads. All are remarkable for a structural 
peculiarity which distinguishes them in a marked manner 
from all other known infusorial forms, such peculiarity con- 
sisting in their possession of a distinctly prolonged anal 
passage, which, produced from the posterior region towards 
the centre of the body, leaves hut a little interspace between 
its termination and that of the conspicuously ciliated oeso- 
phageal passage produced inwards from the opposite or 
anterior extremity. But a slight prolongation of these two 
tubes is required to effect their junction, when, * a complete 
alimentary tract in its most rudimentary form, as possessed by 
the Proctuchous Turbellarians, and other lower Metazoa, would 
he represented. Examples of this genus, as represented by 
Nyctotherus velox and N. Gyoeryanus , are delineated in PL VIII. 
figs. 6 and 7. It is a remarkable fact that this most highly 
organized infusorial type should be found side by side with 
the Opalinidae, in which the alimentary system is present in 
its most rudimentary form, being, in point of fact, en- 
tirely aborted. This seeming anomaly is, however, satis- 
factorily explained in association with the circumstance that 
Nyctotherus has developed a taste for, and partakes freely of, 
the solid meats ; while the Opalinidae, as babes in evolution, 
restrict themselves to the fluid nutriment provided side by 
side in their chosen habitat. The genus Blagiotoma, represented 
( by a single species, P. lumbrici (PL VIII. fig. 12), corresponds 
considerably with Nyctotherus , but is devoid of a conspicuous 
anal passage. It is found in the alimentary tract of the common 
Earthworm. 
The Peritrichous order of the Ciliata, considering its 
extensive limits, comprises comparatively few true parasitic 
species, though, on the other hand, it includes a very large 
number that must be relegated to the category of Messmates, 
or Commensals. All the representatives of Peritricha are dis- 
tinguished by the character of the ciliary system, which is 
limited to a circular or spiral wreath, developed in the anterior 
region, and conducting to the oral aperture. Those types, 
referable to this order, that claim immediate attention are 
comprised within the respective family groups of the ITrceo- 
lariidae, and Ophryoscolecidse. The first- named of these, in- 
