The medusa represents the unattached, motile stage in cnidarians and is found swimming freely in the water column. Swimming is accomplished by muscular contraction of the dome-shaped umbrella, or bell, which forces water out and propels the medusa in the opposite direction. As the umbrellar muscles contract the bell, fibers of the protein collagen in the umbrella are compressed. When the muscles relax, the elastic recoil of the compressed collagen fibers causes the bell to expand and refill with water. Medusae remain largely at the mercy of the prevailing currents and therefore belong to the group of drifting pelagic organisms known as plankton (Plate 14).
Begin with Polyorchis at the top of the page, and color each medusa separately as it is mentioned in the text. Use a very light color for the umbrella. The tentacles have been removed from the front edge of Polyorchis to show the underside of the umbrella. In order to get the transparent effect of the umbrellar dome in Polyorchis and Haliclystus, color the inner structure first, and then apply the umbrella color. The umbrella of Pelagia is thick and not as transparent as those of the other medusae, so the underlying structures are not shown.
The medusa Polyorchis has a high-domed umbrella that is transparent and clearly reveals the organs within the umbrellar space. the elongate, mouth-bearing manubrium protrudes through the opening of the umbrella. It opens into the stomach, which itself opens into radial canals. Suspended from beneath these canals are elongated gonads, or reproductive organs. The long, extensible tentacles bear nematocysts that sting and capture zooplankton and other small marine animals. Polyorchis is a relatively common medusa of the bays and estuaries of the west coast of the United States and Canada; specimens with umbrellas over 5 cm (2 in) in height are not unusual.
The squat, milky translucent medusa Aurelia is a jellyfish with no manubrium, relatively short tentacles, and four elongated oral arms that surround the
mouth opening. Aurelia is found, often abundantly, in coastal temperate water of both Atlantic and Pacific oceans. Instead of entrapping prey with its tentacles, Aurelia is a suspension feeder. As it sinks through the water, Aurelia catches plankton in the mucus on the inside of its umbrella. The mucous-covered food is then carried to the margin of the umbrella and scraped off by the oral arms. The arms have ciliated grooves that carry food to the mouth. The umbrellar diameter of Aurelia reaches 15 cm (6 in).
Another typical jellyfish is Pelagia. This animal is quite striking with its purple and translucent gray umbrella often exceeding 75 cm (30 in) in diameter. Pelagia occurs in large numbers along the central and southern California coast. The four oral arms are very maneuverable. A large Pelagia trails its nematocyst-bearing oral arms as much as 2.5 meters (8 ft) below the umbrella, entrapping and subduing small organisms that swim into them. The oral arms contract to deliver prey to the mouth, which is located at the center of the umbrella. In southern California, many Pelagia are swept shoreward during the summer months and are the cause of a large number of the jellyfish stings experienced by bathers.
The small (2.5 cm, 1 in) jellyfish Haliclystus does not drift or swim freely, as do most medusae. Instead, Haliclystus attaches itself to surf grass in the rocky intertidal zone, and to eelgrass in quiet waters. Its small, stalked attachment disc protrudes from the center of the upturned umbrella. The gonads radiate out from the center and give the medusa the appearance of a webbed basket adorned with eight tentacle clusters. The mouth is at the center of the umbrella. Haliclystus feeds on small planktonic organisms and other small animals living on the plant to which the medusa is attached. Some species of Haliclystus are able to absorb pigment from the plant to which they attach, thus changing the color of their umbrella to match their substratum.