THE BRITISH FRESH-WATER PLANARIANS.
9
sexually mature specimens of the former species in March, April
and August, and of P. nigra in April and June.
Some fresh-water planarians are able to reproduce asexually
by transverse fission. A century ago Dalyell (6) observed that
both Planaria alpina and Polycelis cornuta were able to divide
in two, each part becoming a complete animal. As he had never
found cocoons of these species, he thought that this was their
normal method of reproduction. Spontaneous fission has been
observed in Bdellocephala punctata, Planaria alpina, Pl. gonoce-
phala and Polycelis cornuta. Details of this process vary in
different species, but as a general rule, transverse division takes
place in the neighbourhood of the pharynx, sometimes in front
and sometimes behind. The conditions which lead to fission are
sudden changes of temperature (see p. 10), in the case of
Planaria alpina and Polycelis cornuta. Pl. gonocephala fre-
quently divides when the temperature is raised but a deficiency
in the supply of oxygen produces the same effect. In some
cases death follows fission, but often the missing portions develop
and a perfect creature results.
Planarians possess in a remarkable degree the power of repair-
ing mutilations of their bodies, and consequently have been
subjected to experiments even as early as 1778, when Pallas
(28) wrote on this subject. In this connection, in Britain, Shaw
(32) in 1791, Dalyell (6) in 1814, and Michael Faraday (11) in
1832, published results of their investigations. Numerous
Continental and American workers have written since then
on the subject. It has been demonstrated that some species of
Planarians may be divided either transversely, longitudinally
or by cuts in other directions, and the separate parts have each
succeeded in repairing the injury and in becoming what appeared
to be an unmutilated animal. An important factor is a favour-
able temperature, when a quick recovery is made. It was found
that a planarian divided into two longitudinally produced two
symmetrical individuals in a period of 20 days. By making
suitable incisions, it was found possible to produce monstrosities
with two heads, or with two tails (35).
Reactions to Temperature. Freshwater planarians (Tri-
cladida) are remarkably sensitive to changes of temperature and a
small rise causes death in some forms. Species which are able to live