YELLOW-BROWN CELLS OF CONVOLUTA PARADOXA. 441 
were observed, and also the dates of hatching of the larve in 
the laboratory. The undulating line in the diagram repre- 
sents the magnitude (in decimetres) of successive daily tides 
from July 24th to August 30th, and is based on the tide- 
almanac for Trégastel. The shaded band represents approxi- 
mately the extent of the Paradoxa zone. The position of 
the zone is such that it is fully exposed by a fall of 80 deci- 
metres, whereas a fall of 76 decimetres leaves it submerged. 
Where the undulating line falls above the shaded band in 
the diagram the zone is submerged, viz. July 30th to August 
8th,and August 14th to 21st; where the tide-line falls below 
the shaded band the zone is uncovered twice daily during low 
water. Hach black dot records the date on which an ege- 
capsule was found either in the open attached to the weed or, 
and this was the case in the large majority of the records, 
was laid in the laboratory by C. paradoxa kept under as 
normal conditions as possible. Each cross marks the date on 
which a batch of larve hatched out in the laboratory. 
The relation between time of egg-laying and tide is at once 
apparent, and may be stated thus :—Heg-laying occurs only 
at periods when the zone is submerged for six or seven suc- 
cessive days, i. e. during neap tides. 
Hence it is that, during the spring tides, only immature 
animals are to be met with, and that supplies of egg-capsules 
can only be obtained by collecting the submerged weed 
during neap tides or by keeping the animals in the labora- 
tory. 
It must remain for the present an open question whether 
this phenomenon of periodic egg-laying is to be interpreted 
as a physiological consequence of the more active nutrition 
which obtains during exposure to the higher light-intensity 
of the preceding spring tides or whether it must take its place 
in the innumerable crowd of biological phenomena of adaptive 
significance (compare Keeble and Gamble, 1907). It is evi- 
dent, of course, that during the slack tides the eggs, being 
permanently submerged, are protected from the risk of dessi- 
cation, and to some extent from shock to which they would 
