192 SECTIONAL ADDRESSES 
crustaceans, and I have seen the same or similar forms), crawling about 
the entrance, could enter far enough to touch and press inwardly, slightly 
denting the surface of the door in that region. This would upset the 
unstable equilibrium and the pressure of water would take advantage of 
the initial flexure, which would then travel toward the door edge at the 
middle of the middle piece (Fig. 7). ‘This, in turn, would nullify the 
lateral thrusts of the sides of the door, and the water pressure would, 
folding up the sides, open the door (Fig. 3). ‘To be noticed is the fact 
that the structure of the door in the region of actuation is such that it 
would give readily to local pressure, though the distributed water pressure 
would be resisted (Fig. 6). 
In summarising the case before us, we note that the threshold is broad 
(ratio of breadth to length, 2: 1 approximately) ; that the door is long 
and narrow and has no special tripping mechanism, an initial dent being 
sufficient to upset the unstable equilibrium ; and that the velum is very 
broad. The door is held in tight application to the threshold by the 
thick lateral regions, these exerting a downward thrust on the middle 
piece, which can bend longitudinally. ‘The end of the beak also con- 
tributes to the door mechanism, and has inner course cells which are 
strengthened by props. Although simple in appearance, the door 
mechanism is elaborately endowed with suitable curvatures and cellular 
structures which make its behaviour possible. It is a snap-action 
mechanism, as determined by much careful observation. 
U. cornuta is a New World type, and the few species have the identical 
trap structure. A species (my No. 43) from the Aripo Savannah, Trini- 
dad, collected in fluid by Professor R. B. Thomson in 1931, with spatulate 
leaves (thereby distinguishing it from U. cornuta with linear leaves), and 
U. juncea (Vahl) Barnh., collected by De la Cruz, 1543, in British Guiana 
(my No. 108) and by Britton and Britton (29) in Porto Rico (my No. 110), 
are the only other species I know. Of U. juncea I have seen only 
herbarium material. 
Tue Type U. CAPENSIS (Fig. 20). 
I choose for the next type U. capensis, which I had the opportunity 
to study in the living condition on the occasion of the meeting of the 
British Association at Cape Town in 1929. For the use of the laboratory 
and facilities J am indebted to Miss E. L. Stephens, who has continued 
to help me in various ways. ‘The reason for the choice of this type is 
the similarity in the general proportions and curvatures of the mechanism 
we are considering to those of U. cornuta, though it differs in having no 
beak, in the sense we have used the term for that species, nor is the 
posture of the door quite the same. 
The trap (1 mm. long) has a thick, and in front, surrounding the 
entrance, massive structure. It is one with a considerable number of 
species in which the front is provided with a number of radiating rows 
of long trichomes, graduated in size, forming guides to the entrance, 
as we may suppose. These trichomes are glandular and have the typical 
three-celled structure, the basal cell being much enlarged and often sub- 
divided. The number and arrangement are such as to lend to the trap 
