(13) The change in rate of nerve-conduction (pulsation) in activated specimens over a range in temperature from 23° to 33° C. follows nearly a straight line, the actual rate being approximately doubled for this increase of 10° C. Beyond 35° the rate rapidly declines, death ensuing at 38°. The curve for this change resembles very strikingly that for enzyme action. The change in rate of active half-disks is much more erratic. Such specimens can, however, be carried to much lower temperatures (14° C.) before activity ceases, and the upper limit of temperature was always higher by 2 or 3 degrees than for the activated half of the same disk. When a single sense-organ was subjected to changes in temperature, the remainder of the disk, without its sense organs, being maintained at a constant temperature, the results showed no characteristic difference from those obtained when the whole disk was subjected to the temperature change.
(14) As the area of tissue enervated by a single sense-organ is succes sively reduced to one-half its former extent at each operation, the rate of pulsation declines until, when the area has been reduced to 4 its original extent the pulsations will have been reduced to approximately half their original number. Beyond this point the reduction in rate of pulsation followed the same ratio to the area, at least until the final area of its original extent. A piece of subumbrella tissue 0.0001 the original area of the disk is still capable of responding to each induction shock when applied at the rate of 126 per minute. Appar
ently, therefore, the necessity for a latent period on the part of the muscles is not an important factor in the decline in rate of pulsation. It appears more probable (as indicated by the manner in which each of the 16 sense-organs, usually present on a medusa, take up in turn the initiation of the stimulus for pulsation) that some chemical interchange between the sense-organs and the surrounding tissue is necessary in order that the activity of these structures shall be maintained at the highest state of efficiency. As the tissue area is reduced this readjust ment takes place at a correspondingly lower rate, because the material essential for this adjustment is less readily available to the sense-organs.
(15) The evidence from the foregoing experiments establishes the fact that some sort of trophic influence is exerted in general metabolic activities by the sense-organs. The structure of the nervous system of Cassiopea makes it impossible, however, to prove the existence of trophic nerve-fibers as distinct from those of sensory or motor function. On the other hand, it is shown that in this lowly organized form there are transmitted from the nerve-centers certain influences that are abso lutely essential for maintaining general metabolic activities at their normal rate.