There is perhaps no marine animal which excites more wonder than the jellyfish. Its transparency, its graceful rhythmical movements, its long streaming tentacles, the variety and eccentricity of its form, and often of its color, attract attention, and one naturally desires to know something of its life-history. Jellyfishes are also called /medusæ/, because their long appendages suggest the locks of the Gorgon; /acalephs/, on account of their stinging or nettle-like properties; and sun-jellies, sea-blubbers, etc., because they float upon the surface during the warmest part of the day, when the sun is high. The name /jellyfish/ is inappropriate, since the animal in no way resembles a fish except in the fact that it swims; but it is, nevertheless, the commonest name. Jellyfishes vary in size from that of a pinhead to six or seven feet in diameter. They differ in the number, size, and position of the tentacles, the number of the radial canals, the form of the manubrium, the position of the egg-sacs, etc.; but the general plan of the internal structure is the same in all species. In shape they are compared to a mushroom. From the center of an umbrella-like top falls a central organ like the stalk of a mushroom. It is called the /manubrium/ and is the mouth and stomach of the animal. From the top of the manubrium radiate straight or branched tubes, which are connected with a canal which runs around the whole margin of the umbrella. Extending around the inner circumference of the disk in certain species (usually the hydroid medusæ), there is a horizontal shelf, called the /velum/, or veil, because it sometimes falls like a veil. [pg135] From the margin of the umbrella depend the tentacles. There are little mineral deposits, like crystals, called /lithocysts/, disposed at intervals on the margin, and known also as /marginal bodies/, which are supposed to be eyes. In some species these lithocysts are inclosed in club-shaped bodies, and they are then called /tentaculocysts/, because they are like small tentacles. These, together with the nerve-fibers, are called the sense-organs; but to what extent jellyfishes can see and feel is undetermined. This is the first appearance of sense-organs in animals. Around the concave surface of the umbrella is a muscular zone, or zone of contractile tissue, by which the animal opens and shuts the umbrella and gets its locomotive power. The /gonads/, which are conspicuous from being more opaque than the rest of the body, are the egg- or sperm-sacs. They vary in form and in position.