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POPULAR SCIENCE REVIEW. 
eluding the genera Trichodina , TJrceolaria , Cyclochceta , and 
Licnophora , are mostly distinguished for their ectoparasitie 
mode of existence, being in the majority of instances attached 
to the cuticular surface of the various invertebrate hosts they 
patronize, and apparently deriving their nutriment from the 
mucous and waste material cast off by these latter. Perhaps, 
indeed, it would be more correct to refer them to the Com- 
mensal series. 
In order to enable these Urceolariidae to retain a firm held 
upon their elected fulcrums of support, the posterior region of 
the body is so modified as to constitute an adhesive disc or 
acetabulum, usually strengthened by a complex horny ring, 
over which its possessor maintains so complete a control that it 
can affix or detach itself at pleasure, and lead a sedentary 
or free-roving existence. Among the more familiar examples 
of this highly characteristic group reference may be made 
to Trichodina pediculus , illustrated by Plate VIII. figs. 13-17, 
which often occurs in abundance upon the body and ten- 
tacles of the fresh-water polypes Hydra viridis and vulgaris. 
Other species of the same genus, e. g. T. Steinii , T. baltica , and 
T. scorpoena, are found attached to the cuticular surface of 
respectively a fresh- water Planarian, a Baltic Neritina , and 
the Sea Bullhead, Coitus scorpius. In Cyclochceta , a singular 
modification of this same structural type, long, erect, setose 
filaments are developed in a crown-like manner round the body 
immediately above the acetabulum. The single known repre- 
sentative of this genus, C. spongillce (Plate VIII. figs. 20 & 21), was 
recently discovered by Mr. W. H. Jackson inf esting the cortical 
layer of the fresh- water Sponge, Spongilla fluviatilis, obtained 
from the river Cherwell at Oxford. Licnophora , while some- 
what resembling Trichodina in outward form, is distinguished 
from it by the fact that, the anterior and posterior regions 
of the body are separated from one another by a stalk-like 
intermediate portion, while it is the right, and not the left 
limb of the adoral fringe of cilia, that descends into the oral 
aperture. Both of the two known species, Licnophora Aucr- 
bachii and L. Cohnii , are essentially marine, the former one, 
(Plate VIII., fig. 18), occurring on the Planarian, Thysanozoon 
tuberculatum , in the neighbourhood of Naples. A remarkable 
isomorphic or mimetic representative of the trichodinic struc- 
tural type is afforded by the animalcule represented in Plate 
VIII., figs. 22 and 23, and upon which Mons. Claperede has con- 
ferred the title of Trichodinopsis paradoxa. This species, which 
occurs as a parasite within the intestinal and pulmonary cavities 
of various species of Cyclostoma , corresponds in all respects with 
the typical Trichodince , excepting that the entire surface of the 
body is clothed with fine vibratile cilia. This single circum- 
