Page:Popular Science Monthly Volume 33.djvu/600

582 of water which is discharged by the next muscular pulsation. The tentacles are so elastic and hair-like that they are held by the resistance of the water, and are drawn out behind the animal into fine glassy threads which are thrown into graceful undulations at each pulsation as it swims through the water, and, when it comes to rest and sinks slowly toward the bottom, they form a web or net which is almost invisible, but far more dangerous than any spider's web, for every thread is covered with the terrible poison-darts.

Great as the difference is between the sedentary hydra and the swimming jelly-fish, comparative anatomy shows that they are modifications of the same type, and that the jelly-fish, like the blastostyle, the defensive hydra and the root, is a specialized feeding hydra.

In some species of Dysmorphosa the jelly-fish which is set free from the blastostyle is the last stage in the long series, and it quickly acquires reproductive organs, lays its egg or discharges its spermatozoa as the case may be, and dies; but in other species it no sooner begins its own independent life than it produces buds which are ultimately set free, as jelly-fish like the parent, each of which soon becomes a mature male or female. The eggs are thrown out into the water, where they are fertilized by union with the male cells, and each egg then begins the process of development, which is to result in the founding of a new hydroid colony. The life of the jelly-fish is very short, and simply serves to multiply the species, and to scatter the eggs far and wide along the shore of the ocean, and thus to secure the wide distribution of the hydroids. The egg hatches, however, neither into a jelly-fish like the parent, nor into a hydra, but into a minute microscopic animal of extremely simple structure, which is known as a planula. Fig. 2, a, which is a highly magnified drawing of the planula of another species, will serve to show what it is like. It has no mouth nor tentacles, and its pear-shaped body is covered with cilia, by means of which it swims slowly through the water for a short time, but, unless its slight locomotor power soon brings it into