90 



It is now proper to return to a further consideration of the door 

 with a view to understanding how it is rendered water-tight. We 

 have remarked already that the free edge of the door follows a 

 curious course. It starts on either side where the rear edge of 

 the doorstep merges with the lateral walls and traverses the sloping 

 portions of that structure. Its central portion lies just in front of 

 the doorstep. 



That this of itself would form a water-tight joint has seemed 

 improbable and careful observation has revealed the presence of 

 a thin, almost transparent 'veil' immediately before the door and 

 following closely the anterior edge of the doorstep (figures 2, 5 and 

 10) (Loyd, 1929). Although the free edge of the door starts at the 

 rear edge of the latter the door itself bulges against the veil 

 throughout its length and possesses triangular wings that are closely 

 applied to the lateral parts of the doorstep. In these facts we 

 have, to my mind, a possible explanation of the observed fact that 

 the trap when sprung is not usually distended to the utmost (refer 

 to figure 4). If the veil functions as a valve it would seem that a 

 certain difference of pressure between the interior of the bladder 

 and the outside water is necessary to keep it closely pressed to the 

 door. There is a difference of pressure when the trap is in either 

 the set or the sprung state and it is only when the door is held 

 open or when the bladder wall is pricked that pressure equilibrium 

 seems to exist. 



Such an explanation may appear fanciful but we must remem- 

 ber that the mechanism must approach perfection before it can 

 function in the manner observed. It is interesting to notice that 

 even this remarkable trap fails on occasion and the victim is 

 jammed in the entrance (figure 9). When this happens the trap is 

 unable to function. 



The tripping of the mechanism, leading to the opening of the 

 door and the resultant inrush of water, is brought about, as far 

 as we can tell, by depression of the trigger hairs which pro- 

 ject from the door. Immediately above the point of attachment of 

 these bristles is a thin, easily flexed region of the door and it seems 

 likely that a downward movement of the hairs results in a bending 

 of the door at this point and the lifting of the central portion of the 

 edge from its position before the doorstep. The inrush of water 

 then opens the door but it closes again before equilil:)rium can be at- 



