THE 
ESSEX NATURALIST: 
BEING THE 
^ournaf of f0e (Bsser <ftefb CfuB. 
VOLUME XX. 
THE BRITISH FRESH-WATER PLANARIANS 
(TRICLADIDA). 
BY HENRY WHITEHEAD, B Sc. 
(With Plate I.) 
[Read in abstract 25th February, 1922.'] 
E VERY 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 
