If the rate of consumption of oxygen be taken as a measure of the metabolism of the corals, it appears that the metabolic activity bears an inverse ratio to the coral's ability to withstand the effects of carbonic acid and their ability to resist high temperature follows nearly the same law. It seems possible, therefore, that under the influence of high temperature carbonic acid may accumulate in the tissues faster than it can be eliminated, and acids being toxic would soon cause death.
In the case of the scyphomedusa Cassiopea, I find that for temper atures below 37° there is no time factor--that is to say, if the pulsating subumbrella ring deprived of sense-organs and having an entrapped neurogenic contraction wave be taken from sea-water of about 29° and placed in water of 36° C., it at once assumes a rate characteristic of this higher temperature, and this rate is the same as if the tempera ture had been slowly raised so that two or more hours elapsed before the temperature rose from 29° to 36° C. Moreover, the reaction is completely reversible, the rate returning at once to the original when the ring is replaced in sea-water at 29°. At about 37° or 38°, however, the rate of the nerve impulse begins to decline sharply, and I find that this is subject to a time factor, the decline becoming more and more pronounced as the heat, even though constant in temperature, is continued. Moreover, when the superheated ring is replaced in sea water at 29° C., the pulsation is slower than it was before being sub jected to the excessive heat. A time factor for the effect of extreme heat has been known since the time of Sachs (1865) and has been studied by Blackman (1905) and others. After about an hour, more or less complete recovery of rate may take place, especially if the heat had not been too great or exposure to its influences too long. In
other words, at injuriously high temperatures may accumulate more rapidly than the tissues can eliminate it and the time-factor, according to my hypothesis, is due to this fact.
Thus in a typical case, a nerve-wave in Cassiopea having a rate of 100 at 29.3° had the following rates when heated to 37.8° C.: 100 at 29.3°, 125 at 35.6°, 126 at 36.7°, 127 at 37.3°, 120 at 37.8°. Thus the nerve-conduction became more and more rapid up to 37.3° C., above which it declined sharply, becoming 120 after being for about 20 minutes at 37.8° C. The animal was then taken from 37.8° C. and replaced in sea-water at 29.3° C., and its rate declined at once to 80, thus being 20 per cent less rapid than it was at the beginning of the experiment; but after remaining in sea-water at 29.3° C. for nearly 2 hours the original rate was completely restored.
It will be recalled that Blackman (1905) and Harvey (1911) ad vanced the theory that some enzyme might be destroyed by the exces sive heat, and, being essential to nerve-conduction, its loss caused the rate to decline. It is also possible, however, that some toxic-acid substance is formed under the influence of excessive heat, its rate of formation being commensurate with the metabolism of the tissues. It is easy to see how an acid of this sort might be eliminated and the rate gradually restored when the animal is replaced in normal sea water, whereas if an enzyme were destroyed it might not so readily be replaced.
In any event, one or the other of the above-mentioned hypotheses seems more in accord with the facts than does Winterstein's asphyxia tion theory, or the theory that death from heat is due to coagulation of proteid substances. Death occurs at too low a temperature for coagulation in most if not all proteids; and when killed the animals are fully relaxed, as shown by Harvey. Also, coagulated proteins could not readily be eliminated when the animal was restored to water at normal temperature, coagulation being a practically non-reveisible process.
Summary.
It seems possible that death from high temperature may be due to the accumulation of acid (possibly in the tissues, the rate of formation of this acid being related to the rate of metabolism of the tissues. Thus animals of the same class having a high rate of metabo lism, as measured by oxygen consumption, are more sensitive to heat and to than are those having a low rate of metabolism.