BEHAVIOUR OF CELLS 9 
of rosol—a substance which is decolourized by carbon dioxide, 
and is not injurious to Paramecia—it will be seen that, where 
the groups are collected, the reddish tinge fades and disappears. 
As the groups expand, and are less densely packed, the colour- 
less area expands too: and the limits within which the group 
is circumscribed are also the limits of decolourization. Dr. 
Jennings considers it beyond question that the assembling of 
Paramecia is due to the presence in such assemblages of car- 
bonic acid produced by the animals themselves. The first 
beginning of the crowd may be some small fragment of bacterial 
clot or other substance. . 
It would seem, then, that Paramecia are attracted by faintly 
acid solutions ; and here at least there is, it may be urged, an 
element of choice. But even here, according to Dr. Jennings, 
there is not only no real choice, but not even any real attrac- 
tion. What takes place, according to his observations, is 
briefly as follows. Suppose a faintly acid drop be inserted 
beneath the cover-glass. Paramecia may almost graze its 
boundary without being in any way affected by its presence. 
But in their random movements some, and eventually many, 
perhaps most, of the little animals chance to enter the faintly 
acid region ; but there is no sign of reaction or response ; they 
swim on across the drop until they reach its further margin. 
Here a reaction does take place. Instead of proceeding on- 
wards, slowly revolving on its long axis, a Paramecium thus 
situated jerks backwards by a reversal of all the cilia, at the 
same time revolving on its axis in a direction opposite to that 
in which it was before turning. But the cilia of the mouth- 
groove resume their normal mode of working sooner than the 
others, and this causes the Paramecium to turn aside. It then 
goes ahead until it again reaches the boundary at another point, 
when the same behaviour is seen. The course of such a 
Paramecium is shown in Fig. 2, Iv. 
If, instead of a faintly acid drop, a little alkaline liquid be 
introduced beneath the cover-glass, the Paramecium similarly 
jerks backward and turns aside on reaching its outer boundary. 
The turning may carry it away from the alkali, as shown in 
