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 0°C and
placed in water at 12°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
11°C.
No systematic work on the distribution of this species has
been done in Britain, but on the Continent it has been found that
Pl. 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 Pl. 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 Pl. 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 12°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:—
(1). Zone of Pl. alpina.
(2). ,, ,, Pl. alpina + P. cornuta.
(3). ,, ,, P. cornuta.
(4). „ ,, P. cornuta + Pl. gonocephala.
(5). ,, ,, Pl. gonocephala.
The remarkably wide distribution of Pl. 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