Genus 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
heliozoön _Actinophrys_.


Mesodinium cinctum, n. sp. Fig. 31.