contractile vacuole is posterior and dorsal to the long axis of the
body. The anus is ventral to this axis and also posterior. Length
45µ, width 20µ. Common.

This marine variety is much smaller than the fresh-water form and the
form differs in a number of respects, viz, in the anterior torsion
and in the structure of the mouth. These may be, however, only
individual variations of a widely spread species, and I believe it is
perfectly safe to describe this as _Colpidium colpoda_.


[Illustration: Fig. 38.--_Colpidium colpoda_.]


Genus URONEMA Duj. '41.

(Quennerstedt '69; Cohn '66; Kent '81; Bütschli '81; Schewiakoff '89;
Shevyakov '96.)

Minute forms; colorless and constant in body form. The form is oval,
slightly compressed on the ventral side, while the dorsal side is
distinctly arched. The membrane is distinctly marked by rather widely
separated striæ. These occasionally have a spiral course about the
body; in all cases they can be easily counted. The mouth is large
and placed near the center of the ventral surface. It is sometimes
approached by a very shallow depression or peristome from the
anterior end, and marked by two rows of cilia. An undulating membrane
extends down the mouth. Oesophagus absent. A long, stiff bristle
extends outwards from the posterior end. The contractile vacuole is
terminal or subterminal and near the anal opening. The macronucleus