Zoochlorella.
When not contracted, the peristome end is widespread, rarely
narrowed. The adoral zone and peristome agree with the details given
in the family characteristics. The chief character is the attachment
of the posterior end by means of a single, longer or shorter, stalk,
which contains a highly contractile thread easily distinguished
in the living animal. Another character is the absence of colony
formation. Contractile vacuole, single or double, usually connected
with a sac-like reservoir. The macronucleus is invariably long and
band-formed, with attached micronucleus. Fresh and salt water.

So many species of _Vorticella_ have been described that the task of
collecting data and of arranging the synonyms is extremely irksome
and difficult. Stokes enumerates 66 species, inhabiting fresh and
salt water, and several other new species have been added since his
work. I am impressed with the fact that new species have been created
without proper regard for the manifold variations which nearly all
of the _Ciliata_ show, and I believe the 66 species might be safely
reduced to 12 or 15.

Vorticella patellina Mueller. Fig. 59.

Body campanulate, widest at anterior border, from which it tapers
directly to the pedicle. The diameter of the peristome is a little
larger than the length of the body. The ciliary disk is but little
elevated. The cuticle is not striated and the body plasm is quite
transparent. Length 52 mu.

[Illustration: Fig. 59.--_Vorticella patellina_.]

Vorticella marina Greeff. Fig. 60.

The body is conical but variable, and may he short or elongate, so
that relative length and breadth offer no chance of identification.
In general the body is campanulate. The distinguishing feature is the
transverse annulation of the bell.

Small, but common, and grows in small social groups. Length 35 mu.

[Illustration: Fig. 60.--_Vorticella marina_.]

Genus ZOOTHAMNIUM (Bory de St. Vincent 1824) Stein '38, '54.

Colorless and highly contractile forms growing in small or large
colonies. The form and structure of the individuals is not different
from _Vorticella_. The colonies are usually richly branched upon
the dichotomous plan and the entire colony is contractile. The main
character is that with each division of the individual the stalk
also divides, each daughter cell getting one-half of the parent stem.
The stems therefore remain in communication, so that a simultaneous
contraction results,