THE BRITISH FRESH-WATER PLANARIANS. 11
Pl. alpina was ubiquitous at the end of the Glacial period, and
that as the climate became warmer it survived only in the higher
and cooler portions of the river systems. As it vacated a particu-
lar section of a stream, its place was taken by P. cornuta. This
explanation is supported by the facts of the distribution of the
species in regions where there are glaciers now.
Another theory seeks to explain the distribution of Pl. alpina
by the fact that the animal shows a marked tendency to move
upstream against the current.
Von Hofsten (15) has found the species abundant in the
streams of N. Sweden. He does not consider the species to be
a glacial relic. To give only one of his reasons, he found Pl.
alpina in situations which it could not have reached by active
migration.
Enemies. The slimy excretions render planarians unpalat-
able to many creatures, but it has been recorded that they are
sometimes eaten by newts, sticklebacks, and certain carnivorous
insect larva;.
Mr. John Ritchie, junr. (31), found the parasitic infusorian,
Trichodina steinii (C. and L.), plentiful on the bodies of Planaria
gonocephala. I have found T. steinii moving freely over the
surface of the body of the Rhabdocoel, Mesostoma tetragonum.
Apparently the host suffers no inconvenience. An allied species
of Trichodina is found on Hydra viridis.
CLASSIFICATION AND IDENTIFICATION.
British Species of Fresh-water Planarians.
It is difficult to draw up a list of British species from the very
scattered and scanty records available. To the first list given
below, containing British species already recorded, is added
a second, of species which are widely distributed and which
very probably exist in Britain but have not yet been recognized.
I. British Species already recorded.
Bdellocephala punctata (Pallas).
*Dendrocoelum lacteum (Muller).
Planaria torva M. Schultze.
*Pl. polychroa O. Schm.