The creature pictured is not a protozoan. It is a rotifer. Rotifers
belong to a group of metazoan animals known as pseudocoelomates or
aschelminthes. This group is composed of about 10 phyla including the
Rotifera. Other phyla in this group include Nematoda and Acanthocephala,
both well known to parasitologists.
Rotifers are free living organisms found primarily in fresh water. They
may be found in every conceivable fresh water habitat. They survive
drying and quickly colonize even temporary bodies of water. Despite their
small size, these creatures posses a nervous system, eyes, mouth,
stomach, intestine and musculature. They reproduce by parthenogenesis and
lay eggs.
The rotifer body is divided into corona, body and foot. The corona
contains two groups of cilia which beat resembling rotating wheels. In
the photograph, just below the corona their is a structure that looks
like a paraenthesis within a parenthesis, this is the mouth. The body
contains the digestive canal and reporductive organs. The foot, at the
posterior end often bears projections or toes for attachment to a
substrate.
Rotifers are the most common metazoan animals on earth. One rotifer,
Philodina sp. (which may be the organism pictured) is universal in its
distribution.
