MEDUSA (Lat., from Gk. 314Souo-a, Medomsa, name of one of the three Gorgons, from plocip, niedeia. to rule). A general name applied to the disk-like, umbrella-shaped jelly-fish, with long marginal feelers, and so called from their re semblance to the fabled 1\lednsa's head. (See Plate.) While the term medusa is now generally applied to the sexual free-swimming adult stage of any hydroid, it is particularly applicable to our common North Atlantic Aurelia flariduta of the class gryphozoa (the group formerly called Ifiseophora). Another general name is aealeph.
Our most abundant medusa is Aurelia flavi Milo, which late in summer abounds along the coast from New York northward. It grows to the diameter of from eight to ten inches. becoming fully mature in August. Its rather Tough jelly like disk is moderately convex and evenly curved, while four thick oral lobes delamd from between the four large genital pouches; the edge of the disk is minutely fringed to the ends of the ten tacles. On the fringed margin are eight eyes, each covered by a lobule and situated on a peduncle, and occu pying as many slight indentations, dividing the disk into eight slightly marked lobes. The subdivisions of the water-vascular ca nals or tubes are very numerous and anasto loose at the margin of the disk, one of them being in direct com munication with each eye-peduncle. When in motion the disk contraets and expands rhythmic ally, on the average twelve or fifteen times a minute. On the approach of danger the animal below the surface. Though it has lasso cells, it is not poionous to bathers, while the great. Omura aretiou is very much so.
Aurelia spawns in summer, the fe males being distin guishable by t heir yellow ovaries. the corresponding male gland rose ate, 'while the ten tacles of the female are shorter and thicker than in the males. The eggs Pass out of the mouth into the sea along the channeled arms, and in October the ciliated gastrula (Fig. 2) becomes pear-shatuA and at tached to rocks, dead shells. or sea weeds, and then assumes a hydra•idniped Scyphistoma stage ( Fig. 3), with often twenty-four very long tenta
Iles; in this stage it remains about eighteen months. Toward the end of this period the body increases in size and divides into a series of cup shaped disks. These saucer-like disks arc scal loped 1111 the upturned edge. tentacles lead out, and the animal assumes the Strohila stage (Fig. 4). Finally the disks separate, the ripper one becomes detached and with the other disks swims away in the Etohyra form (Fig. .5). When about a fifth of an inch in diameter, and toward the middle or end of the summer,th is young medusa becomes au adult Aurelia.
other forms of greater beauty occur in the Medi terranean and the tropics. A much larger kind of medusa than Aurelia. Cyan, tt arctiett. is common on the t;rand Banks and off the coast of Northern New England. it sometimes attains a diam eter across the disk of from three to even live feet, though it is produced front a Scyphistonnt not more than half an inch in height. It, tentacle-. stream behind, sometimes to the length of several fathoms, and poison the hands of fishermen. Spe cies of Pelagic do not un dergo an alteration of gen erations (see PA NT ENO. but grow directly from the egg. without passing through a Strobila stage.
For various Mediterranean and tropical forms, see Colored I'late of MEDUSA; AND Strut )NOPI1ORE.
Sleduste shelter various kiwis of animals, which live as fellow-boarders or commensals, viz. tem porary non-attached parasites. Sonic of them live in or under the mouth-cavity or between the four tentacles of the larger Such is the little amphipod crustacean, llyperia, which lives within the mouth. while small lishes, such as the butterfish. swim under the umbrella of the larger jellydishes, Cyanea. etc., for Shelter and prOtee small 811111111.4 Of Various the larger jelly-fishes kill by means of their net tling organs small milli,. fishes and true fishes. the animals being paralyzed by the pricks of the minute barbed darts. See OF.1.1:NTEnATA ; ('Ti NI Win \ orl'`.T.