190 SECTIONAL ADDRESSES 
free the threshold, and carefully spread it (which unavoidably tears it, 
spreading the cells a little, but not enough to disturb our observations), 
We may recognise three transverse zones, which I designate the outer 
(o.z.), middle (m.z.) and inner (7.z.) zones, delimited in Fig. 5. These 
zones can be identified with those in Fig. 3, where the threshold is cut 
sagittally. ‘The component cells having the same fundamental structure 
throughout, we find that those of the outer zone are loosely packed and 
that their cuticles are raised up to form a great mass of loose membranes, 
the velum, which, when the door is closed and the trap set, renders the 
door watertight. ‘The middle zone cells, on the other hand, are smaller 
and tightly packed (their outlines, therefore, angular), and their exposed 
faces are flat. ‘This zone is narrowest at the middle and spreads out fan- 
wise toward the lateral limits of the zone, which furnishes a smooth, firm 
surface against which the face and edge of the door can press when the 
trap is set. The inner zone is narrow and lunate, composed of loosely 
packed cells, with more or less irregular contour. Their general surface 
is slightly uptilted toward the outside of the trap. ‘The back edge of the 
threshold is in the form of a roll of tissue, tapering toward the ends. 
I once thought that this furnished a resistant face against which the door 
edge rests, but I was mistaken (1931). This point was finally settled by 
photographing the door and threshold through the wall of the trap while in 
the living, set condition—optically a rather difficult task. 
The door extends from the end of the beak inwardly, is nearly twice as 
long as broad, and contracted at one point (Fig. 4). This point coincides 
approximately with the forward end of the threshold, along the lateral 
margins of which the sides of the door are attached to the wall (d.h., 
Fig. 3). ‘The extreme ends of the free door edge coincide with the 
inner angles of the threshold, while its edge coincides, when the trap is set, 
with the inner border of the middle zone of the threshold. The histology 
of the door, which is composed of two cell courses, is very important if we 
are to understand its effectiveness. The mapping shown in Fig. 4 
indicates four regions. The outer two-thirds, articulating with the wall 
of the trap in the beak, is of cells which have their longer axes transverse 
in the outer course and longitudinal in the inner. All these cells have 
their radial walls strengthened by cellulose props, but these are much 
more numerous and larger in the walls of the inner course of cells. Both 
anticlinal walls of these latter are folded, bellows-like. ‘The inner course, 
as thus constituted, is capable of much expansion and compression. 
Their own proper tendency is to expand, so that a freed door bends 
sharply outwardly. They can bend inwardly in response to pressure on 
the door, however, so that at once they keep the door pressed on the 
threshold, but can be bent inwardly when the door is opened by the 
pressure of water, only to spring the door back into position when that 
pressure is released. Among the outer course cells there is a number of 
gland cells, with oval capitals, which may act as a lure (Figs. 3, 4, 6), 
together with the special oval gland just below the lip of the entrance 
(Fig. 2). The side walls of the door are constituted quite as the forward 
region above described. ‘They (Fig. 4, /.4.) press firmly in lying in con- 
tact with the sides of the threshold (m.z., Fig. 5) and exert a downward 
