1917-18.] Period of Survival of Shore-Crab in Distilled Water. 21 
logically, there is practically no difference in the structure of the membranes 
during any phase of the moult cycle (see Vitzou (15)), though it is possible 
that the post-moult membrane is looser in texture. It has long been known, 
however, that the calcium content of the membranes varies according to the 
moult phase. I conclude from Paul and Sharpe’s estimations that there is 
no calcium in the integument immediately after moulting, and that just 
before moulting the calcium content is much diminished. 
From these considerations it would appear, since the concentration 
of the blood and the structure of the membranes seem to be little altered 
throughout the whole moult cycle, that the duration of survival depends on 
the amount of calcium in the membranes. Within limits, therefore, the 
greater the percentage of calcium, the less permeable are the membranes 
and the longer is the period of survival. 
It is of interest to note that Bayliss (2), on the evidence of several 
researches (see those of Osterhout (8) and Lillie (5)), comes to a similar 
conclusion regarding cell membranes in general, viz. that the presence of 
calcium renders the cell membrane less permeable. Of more interest is the 
result which Meigs (7) obtained with an artificial membrane of calcium 
phosphate. He found that it was impermeable to the chlorides of sodium, 
potassium, and calcium. The observations just quoted further substantiate 
the conclusions already drawn. 
I have to thank Dr J. H. Ashworth and Dr W. W. Taylor for the help 
which they have always willingly given. 
Summary. 
1. The period of survival in distilled water is shortest with post-moult 
crabs, longer with pre-moult, and longest with inter-moult. 
2. Further, a relatively constant percentage of salts is lost, irrespective 
of size ; hence one of the determining factors in survival is the rate of 
loss of these salts. The other factor is the rate of osmosis of water, 
causing cedema. 
3. With regard to the moult cycle, the concentration of the blood and 
the structure of the membranes are seemingly little altered, but the amount 
of calcium in the latter varies with moult-age, being least in post-moult 
and most in inter-moult membranes. 
4. It is concluded, that the higher the percentage of calcium, the less 
permeable are the membranes and the longer is the duration of survival. 
5. (Edema and loss of salts naturally disturb the tonicity equilibrium 
of the body fluid, and thus cause death. 
