ENGLISH ARTICLES

Jellyfish

The popular name of a floating aquatic animal having no resemblance or relation to fishes. True jellyfish are a stage in the life cycle of members of the class Scyphozoa (“cup animals”), of the phylum Cnidaria, a group also called coelenterates. The coelenterates also include the and sea anemones. The jellyfish stage is technically known as a medusa. Medusae consist of a gelat¬inous, inverted, bowl-shaped mass termed the bell, from the margin of which hang down a number of highly extensile tentacles. The mouth is found in the center of the under surface of the bell, and leads into a system of canals in the bell that act as a distributive system for food. Jellyfish can swim by contractions of the bell, forcing the water out below, but their swimming powers are feeble. In general, they are at the mercy of tides and currents, which often pile them up in enormous numbers in bays or cast them up on beaches. Jellyfish are carnivorous animals, capturing with their tentacles any small animals of suitable size with which they happen to come into contact. As in coelenterates in general, jellyfish are armed with stinging capsules (nematocysts) that discharge against prey, injecting them with a narcotizing poison. After it has been stunned by this poison, the prey is carried to the mouth and swallowed. The digested material is distributed through¬out the animal by the canal system. Human reaction to the sting varies, but in most cases, such as with the common sea nettle, Chrysaora quinquecirrha, of North Atlantic shores, it is no more than a burning sensation and a painful welt. A relative of the true jellyfish, however, the Portuguese man-of-war, Physalia physalis, a member of the coelenterate class Hy-drozoa, is particularly dreaded. It has a very painful sting, which can result in death. Jellyfish range in size from micro¬scopic to those, such as Cyanea capillata, with a bell 6 feet (2 meters) or more in diameter.

Medusae are the free-swimming, sexual stage of the jellyfish. The sedentary, asexual stage is called the polyp, which is usually found in colonies attached to various objects. Polyps increase in number by branching and budding off other polyps. Ultimately, the polyps undergo a process known as stro-bilation, in which a number of horizontal divisions form, making each polyp look like a pile of saucers. Each “saucer” is a young medusa, or ephyra, each of which eventually pinches off and swims away. Medusae are either male or female, and those from one colony of polyps are all of one sex. Mature medusae shed their eggs or sperm into the water. A fertilized egg develops into a swimming larva called a planula, which settles on some object and grows into a polyp.

Medusae occur in all seas, chiefly in coastal waters, but some species inhabit the open ocean and others the deep seas. A few kinds, such as Craspedacusta sowerbyi, which reaches a diameter of 3/4 inch (19 mm) and is found in lakes in the United States, live in fresh water.