rior 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 heliozooen.

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

The chief features by which this species is distinguished from the
frequently described _M. pulex_ of Europe are the number of anterior
cirri and the ring of true cilia in place of the central girdle of
cirri. The European form is described with four anterior bristles;
the present form has from 28 to 32. The radial cilia differ decidedly
from the more powerful cirri and they are not in one plane, so that
counting is difficult; they are not closely set. The presence of
tentacles makes these forms of especial theoretical interest,
especially in the light of the origin of _Suctoria_.

[Illustration: Fig. 31.--Side and top views of _Mesodinium cinctum_.]

Genus TIARINA R. S. Bergh '79.

(Claparede & Lachmann '58.)

Body subcylindrical, pointed posteriorly, two and one-half times
as long as broad; encased in covering composed of separate pieces
arranged in five girdles. The pieces bear processes which rest
against neighboring pieces of the girdle. Mouth large, anus terminal
near contractile vacuole. The macronucleus is simple and round. Salt
water.

Tiarina fusus (Cl. & Lach.) emend R. S. Bergh.

Synonyms: _Coleps fusus_ Cl. & Lach. '58; Daday '86; Moebius '88,
Lauterborn '94; Shevyakov '86.

This form, which resembles _Coleps_ rather closely, was placed as a
separate genus by R. S. Bergh. The skeletal parts consist of five
zones of needles composed of an organized substance and embedded in
the cortical plasm, the last zone coming to a point at the posterior
end. The needles have lateral processes, which give a latticed
appearance to the casing. The cilia are long, with a speci