6 M. F. Miiller on the Systematic Position of the Charybdeidse. Fig. 6. Cypris Verreauxii, on its side, magnified 20 diameters ; 5 a, the same, seen from underneath, magnified 20 diameters. Fig. 6, Cypris YaUahensis, on its side, magnified 40 diameters ; 6 a, the same, seen from underneath, magnified 40 diameters, with natural size annexed. 11. — On the Systematic Position of the Charybdeidse. By Fritz Muller*. EscHSCHOLTz's section of the Discophora phanerocarpce formed a well-defined group of closely allied animals, united by a great number of common characters : — the disk a shallow and smooth segment of a sphere, but capable of being more strongly arched during natation, with a notched margin, in the notches of which, always to the number of eight, are the marginal corpuscles with crystals insoluble in acids; round the mouth four arms, and alternating with these, in peculiar pits, the sexual organs, form-ing bowed bands folded like frills ; the stomachal filaments in the same place, and so forth. The mouth, indeed, was sometimes freely open [Medusidce) and sometimes closed, and, instead of it, numerous orifices on the arms {Rhizostomidce) ; but this pecu-liarity of the RhizostomidcBf important as it certainly is for their mode of obtaining nourishment, did not disturb the morpho-logical unity of the group, as it is derived without difficulty from the ordinary form of mouth f. Some subsequently discovered somewhat anomalous forms of Medusidce likewise did not pre-judice the unity of the general picture, which they only served to complete J. * Translated from Wiegmann's 'Archiv,' 1861, hy W. S. Dallas, F.L.S. t Gegenbaur (Zeitschr. fiir wiss. Zool. viii. p. 210, note) declares the polystomism of the Rhizostomidce to be a paradox not reconcilable with the general plan of the Meduscs, and even doubts the fact. The fact is easily ascertained, and has lately been repeatedly proved, even by myself. Its explanation also seems to me to be pretty easy. A temporary poly-stomism, if it may be so called, may be easily seen in Hydroid Medusae, where the margins of a much-folded four-lobed oral fringe lie upon each other here and there; Thus also the polystomism of the Rhizostomida: v,i\\ result from the growing together of the membranous laminae which sur-round the arms of the Phanerocarpce. When the orifices of the arms have the form of long slits, often continued into strap-like tentacles, as in a Cephea of the South-Brazilian coast, scarcely any doubt can remain as to this mode of production. It seems more difficult to explain the perfora-tion of the peduncle of the arms, or its " origin with four roots," as occurs in the same Cephea, and, according to Forskal, in C. octostyla. X Such as Nausithoe, K611., with its eight extremely simple sexual glands, and Trichoplea, n. g., with marginal corpuscles in deep niches on the under surface, two inches from the undivided margin of the disk, which measures two spans in diameter. Amongst the older, less accurately-known species. Medusa persea, Forsk. {Rhizostoma, Eschsch.), is certainly to be placed with the " Acraspeda" notwithstanding its undivided margin and large velum.
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