THE BRITISH FRESH-WATER PLANARIANS. Q 
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, PI. 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. io), in the case of 
Planaria alpina and Polycelis cornuta. PI. 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 (n) 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 
