558 
H. V. WILSON 
closure may be watched under the microscope. Each pore lies 
in a field surrounded by long spicules (tylotes) and when expanded 
is large. As in the cases of the preceding species, I restrict the 
term pore to the actual aperture at the surface, using the term 
pore canal for the very short tube which perforates the wall of 
the pore-papilla. I append the following record of observations 
on the closure of selected pores. 
Pore 1. When the observations began at 2.25 p.m., the pore 
canal and pore were wide open. The pore canal steadily contracts 
until 3.20 p.m. The diameter of the canal at this time is about 
one-third of the original size. While the pore canal is contracting, 
the surrounding spicules come closer together (the whole papilla 
contracts) . The mesenchyme cells at 3.20 extend to the very wall 
of the pore canal. Immediately after 3.20, a thin homogeneous 
looking membrane containing no mesenchyme elements suddenly 
extends out across the open aperture. This pore membrane 
(fig. 18, 3.25 p. m.) in a few minutes time closes the pore com- 
pletely. 
Pore 2. The pore canal at 3.30 p.m. had already contracted 
to about one-half the full size, and was in the same condition as 
pore 1 at 3.20. The pore, however, closes in a different way from 
pore 1. The canal steadily contracts until it disappears, at 3.40 
p.m. 
Pore 3. The pore canal contracts to about one-half its original 
diameter. It then is found covered in near its margin by an 
extension of the dermal membrane. This extension constitutes 
zone 6 of fig. 19. From this zone a further extension in the shape 
of a very thin membrane, zone a, formed exclusively by the epi- 
dermis, extends over the more central part of the canal. It seems 
proper to designate zone a as a pore membrane. My record for 
this pore is not complete. Probably *the pore membrane was 
first formed, and the mesenchyme of the dermal membrane later 
-Streamed inwards, forming zone 6. The pore membrane soon 
closes the pore completely. The distinction between zones h and 
a is later lost, since granular amoebocytes and microscleres invade 
zone a. Even after this has occurred, on focussing below the 
surface, the wall of the pore canal may be seen. 
