THE
ESSEX NATURALIST:
BRING THE
Journal of the Essex Field Club.
VOLUME XX.
THE BRITISH FRESH-WATER PLANARIANS
(TRICLADIDA).
BY HENRY WHITEHEAD, B.Sc.
(With Plate 1.)
[Read in abstract 25th February, 1922.]
EVERY naturalist who has examined plants taken from
a pond has noticed a black creature about half an inch
in length, flat and unsegmented, which glides gracefully over
the surface of leaf or stem. This is one of the commonest and
smallest planarians of our ponds, and is known as Polycelis nigra.
Ponds and streams in this country contain at least eight species
of planarians, the largest attaining a length of two inches. The
colours range from black through brown and grey to white, an
example of the last being another common species—Dendrocoelum
lacteum.
Often enough many people confound planarians with the
smaller species of leeches; both occur in similar situations, viz.,
on the water plants and under the stones in ponds and running
streams, and in mud. The gliding motion of planarians, how-
ever, is quite distinct from the looping movement of leeches.
Planarians, again, are unable to swim; some leeches do so with
vigorous eel-like movements.
The student who makes the acquaintance of this fascinating
group finds the task of collecting information about planarians
laborious and difficult; that must be the justification for this
summary of literature on the subject, with hints on habitats,
identification, rearing and observing.
The name of this little group of animals has been changed
frequently, a fact which is at first confusing: a brief summary,
therefore, recording the more important discoveries, may be of
some use and interest.
A