The New Student's Reference Work/Jellyfish

Jel′lyfish, the name given to free swimming hydrozoa (). They are common at the seacoast and are often seen from the decks of vessels. Some forms look like mushrooms, others have more the form of an umbrella or saucer with something suspended underneath. They formerly were called medusæ and acalephæ. The body is composed of a clear jelly-like substance, often highly colored in parts during the breeding-season. When taken from the water, they soon dry and shrivel in shreds. The bell-shaped swimming discs vary in curvature from that of a thimble to that of a saucer. The animal swims by the contraction of the swimming bell — the water contained inside being thrown out through a circular opening in the membrane. Tentacles and long contractile lines hang suspended from the disc. Jellyfish are provided with numerous thread-cells or nettle-cells, from which thread-like darts can be discharged, and some larger forms can inflict very severe stings by means of these darts. See Romanes’ Jellyfish, Starfish and Sea Urchins.