10 
THE ESSEX NATURALIST. 
through a range of only a few degrees, are termed stenothermous „ 
whilst those able to endure a wider range of temperature are- 
said to be eurythermous. Planaria alpina is an example of a 
stenothermous planarian. If taken from water at o°C and 
placed in water at I2°C., it dies immediately. On the other 
hand, if the temperature be raised slowly, 20°C. may be attained 
before death ensues. This short range of temperature restricts 
the distribution of the species to cool waters with but little varia¬ 
tion of temperature—conditions which, in this country, obtain, 
only in springs and streams in hilly districts. Dalyell (6) found 
Planaria alpina in a spring at Edinburgh Castle, and the writer 
found the species abundant in a spring near Bavelaw, Pentland 
Hills, N.B., where the temperature of the spring in August was 
li°C. 
No systematic work on the distribution of this species has 
been done in Britain, but on the Continent it has been found that 
PI. alpina is confined to springs and mountain streams, except 
in the Alps and in Scandinavia, where the cold water from the 
melting glaciers descends low into the valleys, and in these dis¬ 
tricts the range of PI. alpina is often extended to all the streams 
of an alpine valley. 
Another stenothermous species, Polycelis cornuta has a wider 
range of temperature and is often found in the same streams, 
as PI. alpina, but at a lower altitude where the water is slightly 
warmer. Lower still, the eurythermous planarian, Pl. gonoce- 
phala, is found. This species can withstand a temperature of 
34°C., though asexual reproduction by transverse fission takes, 
place at temperatures higher than i2°C. 
It has been possible in some districts on the Continent to. 
divide a river system from source to mouth into five zones, each 
characterized by the presence of one or two species of planarians,. 
thus :— 
(I). 
Zone 
of 
PI. alpina. 
(2). 
y y 
y y 
PI. alpina + P. cornuta. 
( 3 ). 
yy 
y y 
P. cornuta. 
( 4 ). 
yy 
y y 
P. cornuta + PI. gonocephala. 
( 5 )- 
y y 
y y 
PI. gonocephala. 
The remarkably wide distribution of PI. alpina in springs 
and streams at the sources of rivers geographically wide apart,, 
has given rise to a good deal of conjecture. One view is that. 
