The bell of the Anthomedusae is, as a rule, of greater depth than breadth, conical or a four-sided pyramid in shape, and of a firm consistence 1. Meridional ridges or a network composed chiefly of cnidoblasts may be present on the exumbrella. The velum is broad, The manubrium is cylindrical with a simple mouth and is surrounded by the genital products in the Codonidae: usually quadrangular in cross section in the other families, the genital products being variously arranged in bands. The mouth is furnished with four lobes in the Tiaridae; four solid capitate simple or branched oral tentacles in the Margelidae; and in the Cladone-midae it varies, resembling one or other of the three families named 2. Tentacles to the bell may be entirely absent (Amalthaeidae), or there may be only one (Euphysidae); there are occasionally two, e. g. Gemmaria, Ctenaria, but usually at least four, and when thus restricted in number they correspond to the ends of the radial canals. Their number may be increased in the Tiaridae, Margelidae and some Cladonemidae, the additional tentacles springing from the circumferential canal. They are sometimes grouped in bunches, and in the Cladonemidae are either feathered or branched.
Their root is bulbous, and the eye is situated on this bulb, usually on its outer aspect, but on its inner in those species that habitually carry the tentacles reflexed, e. g. in Lizzia. Most Anthomedusae are small, some Codonidae and Cladonemidae not exceeding 1-2/25 in. The Tiaridae are of fair size, and some species of Turris and Tiara attain a diameter of 1 1/2 in.
The Leptomednsae contrast with the Anthomedusae in the following points. The bell is flat, its breadth greatly exceeding its depth; it is relatively soft, and hence assumes very different forms during contraction, and may even be reversed, as not infrequently happens in Obelia. Exumbrellar ridges are rare. The velum is feebly developed. The manubrium does not project much: the mouth has usually four lobes, often of some length: the atrium is quadratic or polygonal, and the radial canals spring from its angles 3. The latter vary from four (Eucopidae), eight (Melieertidae, &c), to a larger number, e. g. 200 or more, as in Aequorea. They are variously branched in some instances. The genital organs are usually confined to the radial canals but may extend to the manubrium, or be divided, one part lying at the base of the manubrium, the other on the radial canals, as in the Octorchidae. They appear as simple radial bands, divided sometimes into two parts when radial muscles are present in the subumbrella. If long they may be thrown into hollow folds, if short take the form of dependent pouches. The tentacles are usually tubular with bulbous basis, in number 2, 4, 8, 16, or several hundreds.
Other marginal appendages are (1) marginal cirri present in a few genera of all the families, solid filaments scattered between the tentacles, spirally coiled at their extremities, with swollen ends armed with cnidoblasts: (2) marginal bulbs occurring only in the ocellate families and in certain genera, short, thin, with bulbous ends, provided, at least in some cases, with sense hairs: (3) marginal tubercles present in some vesiculate genera, more or less conical eminences tipped with cnidoblasts, usually black, and containing an evagination of the circumferential canal, and frequently opposite to (4) the marginal funnels or subumbrellar papillae which, unlike the preceding, are placed at the base of the velum to the inner or subumbrellar aspect of the circumferential canal. These papillae are conical, hollow, with a terminal pore, and their lining endoderm cells are filled with brown granules and concretions. They are without doubt excretory organs. Ocelli are found in the two families Thaumantidae and Cannotidae, seldom in the Eucopidae and Aequoridae, usually on the outer aspect of the tentacle-bulbs, rarely on the marginal cirri or bulbs, or on the bell-margin itself in great numbers, e. g.
Orchistoma. Otocysts are characteristic of the families Eucopidae and Aequoridae. They lie at the base of the velum, projecting to its outer aspect. The Eucopidae are small in size, averaging (1-2)/5 in., some, however, such as Obelia, do not exceed (1-2)/25-----in. The majority of Leptomedusae, however, range from (2-4)/5 in. and upwards. Some Aequoridae, indeed, are the largest of all Craspe-dota, e. g. Aequorea Forskalea reaches from 8 to nearly 16 inches.
1 In Sarsia the subumbrella separates from the gastral lamina along eight lines so as to form eight closed pouches. See the description of S. tubulosa by F. E. Schulze in his 'Syncoiyne Sarsii,' Leipzig, 1873, pp. 15-16.
2 Complete absence of the manubrium and mouth has been observed in individuals of a Bougainvillea: Mereschkowsky, A. N. H. (5), iii. 1879.
3 The manubrium is exceedingly small in Obelia and Aequorea, and is absent in Staurostoma and Staurophora, in which the mouth is a cruciform slit. Cf. Haeckel, op. cit. ante, pp. 130, 148.
The Medusan eye consists of sense-cells with pigmented (black, brown, violet, red) supporting cells, with basal ganglion cells connected to the outer nerve-ring. A cuticular lens is sometimes present,. e. g. in Lizzia. The otocyst is primitively a groove lined by the ectoderm cells of the subumbrellar side of the velum; certain of these cells contain each a single calcareous otolith, and others connected to the inner nerve ring are converted into sense-cells. Such otocysts are found in Mitrocoma Annae. But in other instances the groove is very deep and the ectoderm cells at its margins proliferate and meet, thus turning it into a vesicle.