406 DR. C. L. WITHYCOMBE ON THE FUNCTION OF 
multicellular hairs (mh). All the hairs from the rim spread out funnel-like, so 
as to lead intruding animals direct to the bladder mouth. Also distributed 
round the rim are a number of club-shaped hairs (fig. 3,a). Scattered over 
the outside of the bladder wall are small, rounded, sessile glands (fig. 5, sg) 
homologous with club-shaped hairs. 
MECHANISM OF BLADDER. 
After “firing,” the lateral walls of the bladder are slightly convex (fig. 2, a). 
The free margin of the valve has sprung back to its position, fitting into the 
groove in the zone of specialized collar cells (fig. 4). A slight amount of 
mucus is secreted, and the bladder is sealed hermetically. The quadrifid hairs 
lining the bladder constantly absorb the fluid within, whether this fluid is 
pure water or an infusion of decaying animals. With reduction in volume 
of contained water the bladder must yield to external pressure ; therefore 
the lateral walls bulge inwards, presenting a concavity externally on each 
Fia. 4. 
Section of bladder, enlarged. x 50. 
side (fig. 2,0). They, however, exert some tension, and would, on release, 
spring back to their former position. The quadrifid hairs continue to absorb 
until equilibrium is reached between the internal negative pressure and the 
osmotic tension, which can be exerted by the cell contents of the quadrifid 
hairs. We have now a considerable tension upon the valve, tending to pull 
it inwards ; but this cannot happen, since the cushion of specialized collar 
: cells prevent the free margin of the valve from moving inwards. The valve 
is also so constructed that it continues to bulge exteriorly at its centre, this 
condition being mechanically more stable than if the valve were plane or 
concave. Were the latter actually the case the valve might easily be sucked 
inwards, its edge slipping over the special cushion of collar cells, 
There is only one movement which can possibly release the valve from its 
catch. This is an upward movement. Such seems to occur when either of 
the four sensory hairs are touched. Once an upward movement has removed 
the tree edge of the valve from its groove (fig. 5), the negative pressure 
