268 
fore, the two membranes which are both regarded as flaps meet 
in a straight line that would be represented by the ridge pole 
of the tent. Wlien the hydranth emerges in the outer world 
for the first time the first cleavage takes place along this line, but 
it continues until there is room for the egress of the hydranth, 
leaving the bottom of both flaps still attached to the hydrothecal 
margin. Mr. P aarm an’s investigation seems to prove that. 
“Sometimes the adcauline piece is attached while the other is free, 
and sometimes the reverse is true. Often the sides of a flap are 
attached for a greater or less distance proximally while they be- 
come free distally, the degree of attachment varying greatly in 
the same species. In most cases both flaps are functional”. 
P aar man and Nut tin g seem to have overlooked, that in Ser- 
tularia pumila 1 ) the adcauline wall is angularly bent from side to 
side and is provided between the two larger teeth with a much 
smaller one, which divides the adcauline sinus into two lateral halves 
but does not reach the free margin of the adcauline membrane 
stretched between the two larger teeth. This membrane which 
must be regarded as the distal part of the adcauline wall is of 
course also angularly bent, and the ridge dividing it into two 
lateral halves arises from the tip of the median tooth. The much 
larger abcauline membrane consists, as the corresponding part of a 
Thujaria and a Diphasia, or as one of the three or four corre¬ 
sponding parts of a Sertularella, of a proximal part, fixed in the 
abcauline sinus, and a distal free valvular part provided with an 
angularly bent margin which fits into the corresponding sinus 
formed by the adcauline membrane. When the hydrotheca is closed, 
the adcauline membrane on account of its thinness inclines a little 
towards the centre of the aperture, and its free margin meets that 
of the adcauline valve, but a perfect closing of the hydrotheca can 
only take place when both membranes are fixed in their corre¬ 
sponding sinusses to the very tips of the teeth, and this is always 
!) PI. IV, tig. 14; 27, pi. 11, figs. 1—8. 
