MESODINIUM Stein '62.

(Maupas '82, '84; Entz '84; Shevyakov '96.)

The main part of the body is globular or conical, with a short,
platform-like oral region, and a deep annular groove about the
middle of the body. The oesophagus is rather long, and smooth or
longitudinally striped. One or more rings of cirri rise in the
groove. If more than one ring of cirri are present, the anterior set
usually point forward and lie close to the anterior part of the body.
The posterior set, on the other hand, cling close to the posterior
region of the body and give to it a peculiar encapsuled appearance.
The most characteristic feature is the presence of four short
tentacle-like processes which can be protracted and retracted from
the oral region. (Mereschowsky says that the entire anterior half
is more or less contractile.) The macronucleus is horseshoe-shaped
or ovoid and is situated in the posterior half of the body. The
contractile vacuole is also posterior.

Movement consists in rapid swimming, with rotation on its axis, or
in creeping by means of its anterior cirri, or in sudden jumping, by
which it apparently clears a distance of 20 times its diameter in one
bound. Mouth parts may also be used for attachment to foreign bodies.
The moving periods alternate with quiescent periods, during which the
organisms with their outstretched and radiating cirri resemble the
heliozooen _Actinophrys_.

Mesodinium cinctum, n. sp. Fig. 31.

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
ante