The Medusæ which bear the name of _Rhizostoma_ have the disk hemispherically festooned, depressed, without marginal tentacles, peduncle divided into four pairs of arms, forked, and dentated almost to infinity, each having at their base two toothed auricles. Such is _Rhizostoma Cuvieri_ of Péron (Fig. 89), the disk of which is of a bluish-white, like the arms, and of a rich violet over its circumference. This beautiful zoophyte is found plentifully in the Atlantic, living in flocks, which attain a great size. It is common in the month of June on the shores of the Saint Onge; in August on the English coast; and along the strand of every port in the Channel they are seen in the month of October in thousands, where they lie high and dry upon the shore, on which they have been thrown by the force of the winds. Such also is _R. Aldrovandi_ (Fig. 90), which appears all the year round in calm weather. It is an animal much dreaded by bathers. It possesses an urticaceous apparatus, which produces an effect similar to the stinging-nettle when applied to the skin. If the animal touches the fisherman at the moment of being drawn from the water, it is apt to inflame the part and raise it into pustules. [Illustration: Fig. 89. Rhizostoma Cuvieri.] _Cassiopea_ and _Cephea_ are two other types belonging to the same group. In _Cassiopea Andromeda_ (Fig. 91), belonging to the first, the disk is hemispherical, but much depressed, without marginal tentacles or peduncle, but with a central disk, with four to eight half-moon-shaped orifices at the side, and throwing off eight to ten branching arms, fringed with retractile sucking disks. _Cephea Cyclophora_, Péron (Fig. 92), is another very remarkable form of these strangely-constituted organisms. * * * * * Having presented to the reader certain characteristic types of Medusadæ, we proceed to offer some general remark upon the organization and functions of these strange creatures. We have, in short, selected these types because they have been special objects of anatomical and physiological study to some of our best naturalists. [Illustration: Fig. 90. Rhizostoma Aldrovandi.]