FIG. 25.—PHOTOCHRONOGRAPHIC APPARATUS.
The [comatula] is habitually fixed to the bottom of the aquarium, just as a plant is fixed to the earth by its roots. It therefore makes nothing but vague motions of the arm, which it rolls up and unrolls; but if the animal be excited by the means of a rod, it will be observed to begin a strange motion which carries it quite a distance. In this kind of locomotion the ten arms move alternately; five of them rise and keep tightly pressed against the calyx, and the other five descend and separate from it. Upon the arms that rise, the cirri are invisible, and while upon those that descend, they diverge in order to obtain a purchase upon the water. These motions of the cirri seem passive, like those of a valve that obeys the thrust of a liquid.
M. Marey says: “I have obtained images of a certain number of other aquatic species, the swimming of the eel, the skate, etc. These types of locomotion ought to be studied methodically, compared with each other, and considered in their relations with the conformation of the different species. It will, I hope, be a new element for the interpretation of the laws of animal morphology, which are very obscure.”
FIG. 26.—MOTIONS OF THE UMBRELLA OF THE MEDUSA.
M. Marey has also investigated the flight of insects by means of chronophotography. These experiments are most delicate and interesting, and the results obtained go a long way towards making up a satisfactory theory of insect life. M. Marey says that the wing in its to-and-fro movements is bent in various directions by the resistance of the air. Its action is always that of an inclined plane striking against the fluid, and utilizing that part of the resistance which is favorable to its onward progression. This mechanism is the same as that of a waterman’s scull (reference of course being to “sea sculling”, and not to “river sculling”), which, as it moves backward and forward, is obliquely inclined in opposite directions, each time communicating an impulse to the boat. There is, however, a difference between these two methods of propulsion. The scull used by the waterman offers a rigid resistance to the water, and the operator has to impart alternate rotary movements to the scull by his hand—at the same time taking care that the scull strikes the water at a favorable slant. The mechanism in the case of the insect’s wing is far simpler. The flexible membrane which constitutes the anterior part of the wing presents a rigid border which enables the wing to incline itself at the most favorable angle. The muscles only maintain a to-and-fro movement. The resistance to the air does the rest, namely, effects those changes in surface obliquity which determine the formation of an 8-shaped trajectory by the extremity of the wing.