5 8 PROTOZOA 



attached to a contractile stalk ; fission takes place in the vertical 

 plane, and one of the two so formed retains the original stalk, 

 while the other swims off (Fig. 60, E^-E^), often to settle close by, 

 so that the individuals are found in large social aggregates, side 

 by side, fringing water- weeds with a halo visible to the naked eye, 

 which disappears on agitation by the sudden contraction of all 

 the stalks. Carchesium and Zoothamnium differ from Vorticella 

 in the fact that the one daughter-cell remains attached by a 

 stalk coming off a little below the body of the other, so as to 

 give rise to large branching colonies. 



In Carchesium (Fig. 51) the muscular threads of each 

 cell are separate, while in Zoothanuiuim they are continuous 

 throughout the colony. Epistylis has a solid, rigid stalk, and 

 may give rise to branching colonies, which often infest the 

 body of the Water-Fleas (Copepoda) of the genus Cydoj-ts. 

 Opercularia is characterised by the depth of the gutter, the 

 height of the collar, and the tapering downward of the elongated 

 disc. Vaginicola, Pyxicola, Cothurnia, Scy2)hidia, all inhabit 

 tubes, some of extreme elegance. Oi^hrydium is a colonial 

 form, found in ponds and ditches, resembling Opei^cularia, but 

 inhabiting tubes of jelly ^ that coalesce by their outer walls into a 

 large floating sphere ; it usually contains the green symbiotic 

 Flagellate Zoochlorella. Trichodina is free, short, and cylindrical, 

 with both wreaths permanently exposed, and is provided with a 

 circlet of hooks within the aboral wreath. It is often parasitic, 

 or perhaps rather epizoic, on the surface of Hydra (see p. 254), 

 gliding over its body ^ with a graceful waltzing movement ; it 

 occurs also in the bladder and genito-urinary passages of Newts, 

 and even in their body-cavity and kidneys. 



II. SUCTORIA = TeNTACULIFEKA 



Infusoria xoith cilia only in the yoimg stated vnthout mouth 

 or anus, hut ahsorhing food (usually living Ciliates) hy one or more 

 tentacles, perforated at the a'pex ; mostly attached, frequently 

 epizoic, rarely parasitic in the interior of other Protozoa. 



^ Of the composition of cellulose (Halliburton, in Quart. Journ. Micr. Sci. 

 XXV. 1885, p. 445). 



- As does tlie Hyitotrichan Kcrona 2}olyporu7n. 

 ^ Permanently ciliate in ITyjyocoma and Suctorclla. 



