CHAPTER VII.
NATURAL RHYTHM.
It will be convenient here to introduce all the observations that I have been able to make with regard to the natural rhythm of the Medusæ. As Dr. Eimer has also made some observations in this connection, before proceeding with the fresh points having relation to this subject, I shall consider those to which he alludes.
In Aurelia aurita, as Dr. Eimer noticed, the rate of the rhythm has a tendency to bear an inverse proportion to the size of the individual. Size, however, is far from being the only factor in determining the differences between the rate of the rhythm of different specimens, the individual variations in this respect being very great even among specimens of the same size. What the other factors in question may be, however, I am unable to suggest.
Dr. Eimer also affirms that the duration of the natural pauses, which in Aurelia habitually alternate with bouts of swimming, bears a direct proportion to the number and strength of the contractions that occurred in the previous bout of swimming. I observed that Sarsiæ are much better adapted than Aureliæ for determining whether any such precise relation obtains; for, in the first place, the strength of the contraction is more uniform, and, in the next place, the alternation of pauses with bouts of swimming is of a more decided character in Sarsiæ than in healthy specimens of Aureliæ. I further observed that in Sarsia no such precise relation did obtain, although in a very general way it is true, as might be expected, that unusually prolonged bouts of swimming were sometimes followed by pauses of unusual duration. As all the observations are very much the same, I shall only quote two of them:—
| Sarsia. | Sarsia (another specimen)., | ||
| Number of pulsations. | Seconds of rest. | Number of pulsations. | Seconds of rest. |
| 54 | 90 | 40 | 60 |
| 20 | 15 | 29 | 90 |
| 9 | 92 | 32 | 132 |
| 51 | 40 | 33 | 92 |
| 38 | 60 | 18 | 59 |
| 1 | 43 | 8 | 63 |
| 63 | 45 | 15 | 35 |
| 1 | 14 | 2 | 85 |
| 60 | 15 | 11 | 63 |
| 6 | 50 | 30 | 33 |
| 38 | 50 | 17 | 81 |
| 22 | 32 | 19 | 67 |
| 25 | 12 | 3 | 65 |
| 56 | 55 | 19 | 36 |
| 65 | 20 | 41 | 123 |
| 42 | 15 | 80 | 23 |
| 35 | 40 | 61 | 150 |
| 76 | 43 | 45 | 145 |
| 40 | 120 | ||
| 10 | 97 | ||
| 14 | 35 | ||
These observations may be taken as samples of others which it would be unnecessary to quote, as it will be seen from the above that there is no precise relation between the number of the pulsations and the duration of the pauses. Nevertheless, that there is a general relation may be seen from some cases in which unusually prolonged pauses occur. The following instance will serve to show this:—
| Sarsia (another specimen). | |
| Number of pulsations. | Seconds of rest. |
| 38 | 30 |
| 22 | 35 |
| 49 | 40 |
| 30 | 45 |
| 46 | 20 |
| 2 | 15 |
| 24 | 380 |
| 112 | 20 |
| 45 | 185 |
| 894 | 30 |
| 6 | 45 |
| 4 | 140 |
| 2 | 185 |
| 30 | 210 |
| 200 | 60 |
In this case, the relation between the long pause of 380 seconds and the subsequent prolonged swimming bout of 112 pulsations is obvious; also, as the latter was then followed by a short pause of twenty seconds and another comparatively short bout of forty-five pulsations, the refreshing influence of the previous 380 seconds rest may be supposed to have been not quite neutralized by the exhausting effect of the foregoing 112 pulsations. At any rate, looking to the general nature of the previous proportions (viz. in their sum 185/211), it is certain that 380/112 leaves a large preponderance in favour of nutrition, which preponderance is not much modified by adding the next succeeding proportion, thus, (380 + 20)/(112 + 45) = 400/157. Consequently, the organism may fairly be supposed to have entered upon the next prolonged period of rest (viz. 185 seconds) with a large balance of reserve power; so that when to this large balance there was added the further accumulation due to the further rest of 185 seconds, we are not surprised to find the next succeeding swimming bout comprising the enormous number of 894 pulsations. But this great expenditure of energy seems to have been somewhat in excess of the energy previously accumulated by the prolonged rest, for this unusual expenditure seems next to have entailed an unusually prolonged period of exhaustion. At any rate, it is plainly observable that the next succeeding proportions are greatly in favour of repose; for it is not until 360 seconds have elapsed, with only twelve pulsations in the interval, that energy enough has been accumulated to cause a moderate bout of thirty pulsations. But next another long and sustained pause of 240 seconds supervenes, and, the animal being now fully refreshed with a large surplus of accumulated energy, the next succeeding swimming bout comprises two hundred pulsations. Lastly, there succeeded sixty seconds of rest, and here the observation terminated.[22]