Sub-Order II. Rhizostomata.
In this sub-order the lips are very much exaggerated in size, and are fused together by their margin in such a manner that the mouth of the animal is reduced to a number of small apertures situated along the lines of suture. Tentacles are absent on the margin of the umbrella. This sub-order contains some of the largest known jelly-fishes, and exhibits a considerable range of structure. The families are arranged by Maas[[360]] in three groups.
Group I. Arcadomyaria.—Musculature of the disc arranged in feather-like arcades. Oral arms pinnate.
Fam. Cassiopeidae.—There are no epaulettes on the arms. Labial tentacles present. Cassiopea is common in the Indo-Pacific seas, and extends into the Red Sea. It includes a great many species varying in size from 4 to about 12 cm. in diameter.
Group II. Radiomyaria.—Musculature arranged in radial tracts. Oral arms bifid.
Fam. Cepheidae.—The genera included in this family differ from the Cassiopeidae in the characters of the group. Cephea is found in the Indo-Pacific Oceans and Red Sea. Cotylorhiza is common in the Mediterranean Sea and extends into the Atlantic Ocean.
Group III. Cyclomyaria.—The group contains the majority of the Rhizostomata. Musculature arranged in circular bands round the disc. Oral arms primarily trifid, but becoming in some cases very complicated. The principal families are:—
Fam. Rhizostomatidae.—With well-marked epaulettes, and sixteen radial canals passing to the margin of the umbrella.
Rhizostoma pulmo (= Pilema octopus), a widely distributed species, is often found floating at the surface off the western coasts of Scotland and Ireland, and sometimes drifts up the English Channel into the German Ocean in the autumn. The umbrella is about two feet in diameter, and the combined length of the umbrella and arms is four feet. The colour varies considerably, but that of a specimen obtained off Valencia in 1895 was described as follows: "The colour of the umbrella was pale green, with a deep reddish margin. Arms bright blue."[[361]]
The family includes Stomolophus, of the Pacific and Atlantic coasts of America, in which the oral arms are united at the base, and Rhopilema, the edible Medusa of Japan and China.