Jelly-fish vary greatly in size. Some are mere dots, so extremely small that we should not notice them in the water, while one species is said to be seven feet in diameter, with tentacles measuring fifty feet (Fig. 3). The parent of this huge jelly-fish was a hydroid only half an inch high. Its children will be the same. What do you think its grandchildren will be?
Fig. 4.—Mushrooms of the Sea.
The size of jelly-fish is greatly enlarged by the water they absorb; indeed, the substance of which they are composed consists largely of water. A specimen weighing several pounds when alive will shrink away to almost nothing if exposed to the sun and the wind. As the body contains no bones or other solid matter, it all perishes together, and no trace is left of its former beautiful shape. You will see that jelly-fish are in no way like real fish. One writer found them so much like a familiar vegetable that he called them "Mushrooms of the Sea."
It would be impossible to describe to you the varied colors of jelly-fish, as they include almost every hue, the beautiful tints being probably due to their transparency. Some are purely white and as clear as glass, while all shades are to be found, from pale blue and pink, to bright red and yellow. Those found in tropical seas are of a deeper color than ours.
In striking contrast with these brilliant jelly-fish is one species which is so delicate and transparent that as it floats upon the water we can scarcely see the substance of which it is composed. The only parts that strike the eye are the circular tube around the edge and the four radiating tubes with their large clusters of eggs. The tubes look as if they were held together by some slight web. The movements of this jelly-fish are languid, and it sometimes remains perfectly quiet in the bright sunshine for hours, not even moving its tentacles.
Fig. 5.—Fixed Jelly-Fish.
You have probably noticed a great difference in the movements of people. So with jelly-fish: some are much more active and energetic than others. While some kinds appear to delight in darting through the water, until one might suppose from their frisky motions that they are having a great deal of fun and frolic, others prefer to make no exertion, and to drift idly with the waves. There have even been found "fixed jelly-fish" (Fig. 5)—those so fond of a settled resting-place that they have put out suckers by which they attach themselves permanently to some rock or stone.
Although jelly-fish are so brilliant in the daytime, they have a different beauty at night, when they throw out a golden light slightly tinged with green, resembling the light of a glow-worm. Vast numbers of small animals in the sea have this power of throwing out light from their bodies. The light is called phosphorescence. As it may be seen at anytime of the year illuminating air oceans, it is an unfailing source of delight to voyagers. It is most conspicuous on a dark night, when the water is agitated by the motion of a boat, or by the breaking-waves, because the disturbance of the water excites the little animals.