ABA FREDERICK KEEBLE. 
of the tides. At the onset of the spring tides the animals, 
chiefly immature specimens, may be found among the fine 
yellow weed attached to Fucus on its seaward edge. During 
the succeeding tides C. paradoxa moves seaward, and must 
be sought among the delicate weeds attached to Ascophyllum, 
whilst still later in the same series of spring tides it is only 
to be found among the Ceramium and Rhodomela and similar 
red weeds which cover the cord-like thallus of Pycnophycus. 
The same weeds permanently submerged a few yards further 
from the shore yield no animals. At both upper and lower 
limit of its zone of distribution C. paradoxa is represented 
only by immature, minute forms. | 
(b) Migrations—Tropisms within this Zone of 
Distribution.—There is, during the large spring tides, 
a tidal migration of Convoluta. The animals follow the 
falling tides seaward, and the rising tides landward. A 
study of the behaviour of the animals in the laboratory 
provides the probable explanation of this “ebb and flow” 
movement. | 
As has. been mentioned, Conveluta tends to let itself be 
carried downward by the water draining off the weed. An 
explanation of the tidal, seaward movement based on this 
fact, sufficient as it seems at first sight, does not account for 
the zonal distribution of the animals, nor for all the facts 
of their ‘ebb and flow” movement. Thus for a little while 
after the water has left the weeds of the Paradoxa zone, 
animals may still be found in fair numbers among them. 
Later C. paradoxa deserts the weeds, and may be caught 
in the act of doing so by catching the drip from them. 
Closer examination of the question shows that a number of 
other factors co-operate in determining these tidal migrations. 
These factors are—(1) response to contact with weed or 
other solid body; (2) behaviour in stillness and darkness ; 
(3) background reaction; (4) phototactic reaction. 
(1) and (2) Response to Contact with Weed, etc.— 
Convoluta exhibits the reaction of thigniotactism, that of 
clinging to a solid object, in a marked yet peculiar fashion. 
