YELLOW-BROWN CELLS OF CONVOLUTA PARADOXA. 435 
The body, fitted to the weed in the manner already described, 
is dislodged from it with difficulty. Nevertheless there are 
conditions under which C. paradoxa relaxes its hold and 
becomes a swimming and no longer a gliding animal. Among 
these conditions the chief are change in light-intensity and 
change in background. Under yet other circumstances C. 
paradoxa relaxes its hold on its substratum, and becomes 
temporarily at least an animal of plankton habit, floating 
passively on the surface-film of water buoyed up by a 
mucilaginous secretion. Stillness of the water and darkness, 
particularly if seaweed is absent, bring about this behaviour. 
Animals placed in a dark vessel exhibit this phenomenon of 
“upness” with uniform regularity. It is not a pathological 
phenomenon, since it is manifested readily by fresh-caught 
animals, and since animals floating thus on the surface-film 
at once respond to stimulation, descending on exposure to 
hight or, and yet more readily, on a slight disturbance of the 
surface of the water. 
The “up” position is assumed whether the upper surface 
of the water is exposed to the air, or whether, contained in 
an inverted glass vessel, the upper surface of the water is 
against the glass. If weed is present in the vessel the 
animals for the most part do not leave it, though occasionally 
some assume the “up” position. Though more marked in 
darkness, this habit of floating on the surface-film is also 
exhibited by animals kept in the hght, particularly if the 
light is feeble. We must conclude that, in the absence of 
any orientating stimulus whatever or of one of sufficient 
intensity, C. paradoxa leaves the “ground,” rises to the 
surface-film, and floats there passively. Like all such move- 
ments, this at the present time is as inexplicable physio- 
logically as ecologically it is obvious. Anticipating the 
evidence (p. 453) proving that C. paradoxa is dependent on 
hght for its existence, we recognise that, borne out of its 
zone by currents or tides, it has, by virtue of this habit of 
“upness,” a chance of regaining its home or of maintaining 
its existence in a new region; without it, condemned to hold 
