Body spherical to pyriform, constricted near the middle, the
constriction dividing the body into dissimilar parts. The anterior
part is broadly pyriform, somewhat plastic and hyaline, with an oral
extremity which is sometimes hollow, sometimes evaginated and convex.
Upon this flexible anterior part there are four short but distensible
tentacles. The posterior part is granular and usually filled with
food particles; it is well rounded and holds the nucleus and
contractile vacuole. The entire body is surrounded by a fine cuticle.
The nucleus is elongate and extends through the greater part of the
posterior half. The contractile vacuole lies on one side, near the
girdle. The mouth is on the anterior pole in the tentacle region. The
motile organs are cirri and cilia, all inserted in the constriction.
There are two sets of cirri and one of cilia; the latter stand out
radially from the girdle and are usually in motion. The cirri of
one set, the anterior, extend forward about twice the length of the
anterior half; those of the posterior set closely engirdle the lower
half, reaching not quite to the posterior extremity. These are
somewhat hyaline and are closely approximated, giving the impression
of a tight-fitting crenulate casing about the lower half. The cirri
are sharply pointed, much broader at the base, and the two sets are
so placed that, looked at from above, they have the appearance of a
twisted cord. (Fig. 31 b.) Movement erratic; sometimes the animal
swims steadily forward with mouth in front; again it shoots across
the field of the microscope, either backward or forward or sideways,
through the action of its powerful cirri. It is often quiet, usually
mouth downward, and is held in place by adhesion of the tentacles. In
this position it looks strikingly like a heliozoön.

Length 35µ; greatest width 30µ. Not uncommon.