Larva of Chaoborus Crystallinus ( de Geer). 
357 
air to escape. If the pressure is removed before this stage is 
reached, the threads will slowly restore themselves to the normal. 
We have tried this out, placing a sac in water in a live box, 
applying pressure until a slight rupture has occurred, the greater 
portion of the air escaping through the aperture. By gently re- 
moving the pressure we witnessed a slow expansion and sucking 
in of water, until more than half the sac was filled. 
Krogh has put forward the hypothesis that water may be 
secreted, and then disengaged, or by a process of osmosis expelled 
from the sacs, causing the creature to ascend and descend in a 
similar manner to that of a submarine, with its tanks filled or 
emptied of water. 
We have great difficulty in entertaining this. Such experi- 
ments as we have been able to carry out, compelling the ‘creature 
to rapidly ascend and descend — it remaining stationary at the end 
of each movement — excludes the idea that any such secretion of 
water can take place so rapidly as indicated, bringing about an 
instant change of conditions enabling a larva to establish its 
equilibrium. We hold to the opinion that, owing to its structure, 
the sac is rigid, and its action under pressure, when compared 
with the soft and somewhat flabby gas bladder of a fish, would be 
different. No alteration of its area takes place with a change of 
position. On swimming downwards there arises a difference in 
hydrostatic pressure between the inside of the sac and the surround- 
ing fluid ; in other words, diminution of pressure in the sac is set 
up, ami a passing in of a portion of available gas takes place from 
the four tubes attached to each of the four riyid sacs (these tubee 
we will describe later), and buoyancy is not disturbed. On 
ascending, the pressure in the sacs being increased, the contained 
gas would find a natural outlet through the capillary tracheae, 
replacing exhausted air due to movement. At the same time, gas 
in the tubes is being replaced, and equilibrium is, therefore, 
maintained. 
The process of respiration is most active at the same period as 
the rapid up-and-down movements in the water. These move- 
ments being limited, the varying pressures would, in consequence, 
be small, and the volume of air taken in and expelled from the 
four sacs at the completion of each change of position would be 
infinitesimal. 
It is well, ho \ever, to bear in mind that Corethra, under normal 
conditions, will remain stationary for long periods, and does not 
indulge in active movements — not even to chase living organisms 
when hunger must have been acute, we having kept it without 
food for days. By this habit of remaining stationary it has learned 
to exist on a minimum amount of oxygen. 
Lieut. G. C. C. Dimant (12) kindly drew our attention to a 
statement made by Wesenberg-Lund in the Internationale Revue — 
2 B 
