44.0 FREDERICK KEEBLE. 
ally during the large spring tides, by a periodic change in the 
amount of light to which it is exposed. As the tide is falling 
off the upper limit of the Paradoxa zone the animals are 
exposed to an increasing intensity of light and to an increas- 
ingly lighter background. The latter factor causes them to 
release their hold on the weed, and so to let themselves go 
with the water draining from the now exposed weed. Other 
animals turn their negative phototactism to a like account 
and, failing escape in the way just indicated, creep into the 
thickness of masses of weed and attach themselves firmly in 
obscure situations on the dark ground which the weeds pro- 
vide. ‘Those which follow down the current will be arrested 
by the weeds of a lower level; thigmotropism and darker 
background will tend to check their course and high light- 
intensity to drive them on. The position assumed will be the 
physiological resultant of these opposing forces. The return 
movement with the incoming tide will result in the animals 
once again taking up the most favoured light-position. This 
most favoured light-region will shift seaward with the 
increasing spring tides, and return again landward as these 
tides fall off. 
(c) Periodicity of Hgg-laying.—The periodically 
changing conditions under which C. paradoxa lives induce 
rhythm not only in the migrations of the animals within the — 
Paradoxa zone, but also in the period of egg-laying. 
Animals were collected daily, whenever the tides allowed 
the Paradoxa zone to be approached from the shore, during 
the months of July and August, 1907. The. results showed 
many minute, mid-sized, and large animals, but scarcely any 
mature females. That the eggs are laid within the zone there 
is no doubt, since, occasionally, capsules are found attached 
to the weed of the zone. 
The scarcity of mature females is due to the fact that 
maturity is reached and egg-laying effected only ens cer- 
tain tidal periods. 
The results supporting this conclusion are displayed in 
text-fig. 8, which records the dates on which egg-capsules 
