THE DEVELOPMENT OF ALOYONIUM DIGITATUM. 51 
attempt to settle), but usually they get caught up here in 
their own mucus and eventually degenerate. 
Some larvae develop more quickly than others, but usually 
on the fourth free-swimming day, i.e. the seventh day of 
development, many larvae settle. After hovering motionless 
for some time with the broad anterior pole apparently touching 
the chosen place for settling, the planula becomes attached 
(Text-fig. 9). In this they agree with Sympodium and 
Clavellina ( 9 ). A thin disc of opaque white mucus (the 
mucous plug) fastens them to the substratum (PI. 3, fig. 3, 
M. P.), this mucus being secreted by the mucous cells in the 
ectoderm of the anterior end. In bowls containing water 
only, the circulation set up by the constantly varying 
temperature of the laboratory carried many larvae to the top 
of the water so that on becoming sluggish they were caught 
up in the surface film and settled there, either on the film 
itself or on the glass wall of the dish (cf.de Lacaze- 
Duthiers ( 10 ), PI. xiii, fig. 6). In the former case the settled 
polyps also developed perfectly, hanging upside down from 
the film until this was disturbed, sending them down to the 
bottom. 
In a certain number of bowls, wherein small Pecten shells 
were placed, the larvae settled in great numbers on both 
surfaces of the shells and on the parts of the dish sheltered 
by them, i.e. the base and lower part. In these cases the 
circulation is modified by the presence of the shells, so the 
larvae are less numerous in the surface film. In some dishes 
the planulae did not settle until the fifth, sixth, to fourteenth 
day of free-swimming life, and although these are results 
obtained under laboratory conditions, a varying length of 
free-swimming life would obviously help dispersal in the sea, 
and give more larvae a chance to find Ascidians, Chaetopterus 
tubes, Hydroids, Pecten shells or other suitable objects to 
settle on. The planulae often settled so close to one another 
as to render a group difficult to distinguish with the naked 
.eye from a young colony. All the larvae did not settle, and 
