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Transactions of the Society. 
gland for fishes living placid lives in inland lakes and rivers, 
experiencing little or no change of pressure in the bladder.” 
Experiments that have been carried out on Corethra , subjecting 
it to varying pressures from + 33 cm. to 2 atmospheres, the latter 
being equal to 10 m. of water, recovery of equilibrium has taken 
place after periods of from one to twenty hours ; but Krogh states 
equilibrium was seldom re-established if a pressure in excess of 
4- 70 to +80 cm. was employed. 
The larvae we have captured have been taken from shallow 
ponds, none of them having a greater depth than 5 feet, and at the 
edge, within the limits of the reach of a hand net, the depth should 
be about 2 to 3 feet. It is around the edges of these ponds we 
have landed our catches. 
It is obvious that, under such conditions, should the larva 
descend to any part of the bottom — which is its invariable custom 
on being scared — the amount of pressure it would sustain would 
be inconsiderable, and, owing to the nature of the construction of 
the air sacs, no compression would take place. 
If, for experimental purposes, the larva is subjected to abnormal 
pressures, the question arises as to what damage the air sacs have 
sustained. If this is considerable, the natural resilience of the 
sacs will be to some extent temporarily destroyed, and until the 
normal shape or form of the sac has been re-established equilibrium 
is delayed. The period for recovery would be controlled by the 
extent of the damage inflicted ; if this is beyond repair the larva 
sinks and remains at the bottom. If the pressure should be 
sufficient to burst the sac, driving the air out, with the removal of 
the pressure the sac could ultimately expand, and we should then 
probably find it practically filled with body fluid. 
As we have seen, the closely-packed hoops of the sacs are made 
very resistant by a ribbon of tissue bound spirally round them. 
When pressure is applied, a definite break of the hoop is avoided, 
but they will buckle as the pressure is increased. (PL XIX., 
fig. 25.) Ultimately, if the strain becomes too great, the in- 
between tissue holding the hoops will burst, allowing the contained 
EXPLANATION OF PLATE XVIII. 
Fig . 17. — Air sac : at one end knob-shaped valve, the other is attached to the 
capillary trachea. 
Fig. 18. — Fractured hoops : produced by pressure in live box. 
Fig. 19. — Complete hoop with spiral bands (transverse section). 
Fig. 20. — Enlarged conchoidal-shaped sac prior to pupation, showing distri- 
bution of pigment spots over entire surface. 
Fig. 21. — Entire longitudinal section of air sac, showing depth of hoops in 
middle of sac, which diminishes as the ends are approached. 
Fig. 22. — Appearance of hoops under polarized light ; overlapping edges of 
spiral bands as dark lines across the hoops. 
