This order contains not only by far the greater number of the species of Scyphozoa, but those of the largest size, and all those that are familiar to the seaside visitor and the mariner under the general term jelly-fish.
They may be distinguished from the other Scyphozoa by several well-marked characters. The umbrella is flattened and disc-shaped or slightly domed, but not divided by a coronary groove. The perradial angles of the mouth are prolonged into long lips, which may remain free (Semaeostomata) or fuse to form an elaborate proboscis (Rhizostomata).
Sub-Order I. Semaeostomata.
In this sub-order the mouth is a large aperture leading into the cavity of the manubrium, and is guarded by four long grooved and often tuberculated lips. The margin of the umbrella is provided with long tentacles.
Fam. 1. Pelagiidae.—Semaeostomata with wide gastric pouches, which are not united by a marginal ring sinus. Pelagia, which forms the type of this family, has eight long marginal tentacles. It develops directly from the egg, the fixed Scyphistoma stage being eliminated.[[359]] It is probably in consequence of this peculiarity of its development and independence of a shore for fixation that Pelagia has become a common and widespread inhabitant of the high seas. In the Atlantic and Indian Oceans P. phosphora occurs in swarms or in long narrow lines many miles in length. It is remarkable for its power of emitting phosphorescent light. In the Atlantic it extends from 50° N. to 40° S., but is rare or absent from the colder regions. P. perla is found occasionally on the west coast of Ireland. Chrysaora differs from Pelagia in the larger number of tentacles. There are, in all, 24 tentacles and 8 statorhabs, separated by 32 lobes of the margin of the umbrella. C. isosceles is occasionally found off the British coast. It passes through a typical Scyphistoma stage in development. Dactylometra, a very common jelly-fish of the American Atlantic shores, differs from Chrysaora in having sixteen additional but small tentacles arranged in pairs at the sides of the statorhabs.
Fam. 2. Cyanaeidae.—Semaeostomata with eight radial and eight adradial pouches, which give off ramifying canals to the margin of the umbrella; but these canals are not united by a ring-canal. The tentacles are arranged in bundles on the margin of the deeply lobed umbrella.
The yellow Cyanaea capillata and the blue C. lamarcki are commonly found on the British coasts.
Fam. 3. Ulmaridae.—The gastric pouches are relatively small, and communicate with a marginal ring-canal by branching perradial and interradial canals and unbranched adradial canals.
In Ulmaris prototypus (Fig. 143, p. [315]) there are only eight long adradial tentacles, and the lips of the manubrium are relatively short. It is found in the South Atlantic.
Aurelia is a well-known and cosmopolitan genus, which may be recognised by the eight shallow lobes of the umbrella-margin beset with a fringe of numerous small tentacles.