NO. 420.] HISTORY OF STICHOSTEMMA. 993 
in tiding over the dry seasons by becoming encysted, and will 
then become capable of living in shallow, temporary pools, 
which dry up during a part of the year. 
At present, however, the formation of the slime coverings 
cannot be regarded as an adaptation to particular conditions. 
It is merely an incidental result of certain reactions of the 
animal which have no relation to lack of water. In those cases 
where the animal leaves the water the result is always or nearly 
always death. If the nature of the secretion should for any 
reason undergo change in some individuals or races, selec- 
tion of those secreting the most impermeable covering might 
occur. 
The question as to whether the emergence from the water 
ever occurs under natural conditions requires a brief consid- 
eration. It might be supposed that in nature the animals can 
always satisfy the thigmotactic impulse among the vegetation 
and débris beneath the surface ; but since the reaction is so 
distinct, and includes so large a proportion of the animals 
in clear water in the laboratory, I can scarcely believe that it 
does not occur sometimes in nature, and probably very often. 
Moreover, as is noted below, foulness of the water often induces 
the emergence, and in the lagoon where these worms are found 
the water in certain localized regions may become stagnant to 
a considerable degree in consequence of the decay of the luxu- 
riant vegetation. I believe, therefore, that the animals must 
leave the water frequently in nature, and that these individuals 
are usually eliminated. It is possible that an animal which has 
left the water may occasionally find its way back to it, but 
I have never been able to observe a case of this kind in the 
laboratory, though I have often looked for it. 
It appears probable, however, that the change from an 
aquatic to a terrestrial habit may be due, at least in such forms 
as the turbellarians and nemerteans, to a positive thigmotactic 
reaction which leads the animal to force itself into the angle 
between the surface film of the water and the substratum 
and thus to leave the water in the manner so well illustrated 
by Stichostemma. Perhaps Stichostemma itself will become 
able in time to resist evaporation and to obtain its food under 
