The tentacles are usually numerous, and may even be disposed in several rows one above the other. Eight, four radial and four interradial, occur in some instances, four radial in Petasus alone, reduced to two in Dipetasus. The primitive tentacles are always solid, but radial hollow tentacles communicating with the circumferential canal are present in Olindias and Pectanthisy and appear in the Geryonidae in the course of growth. Those of the Pectyllidae terminate in suckers l. The solid tentacles have the usual structure: their cnidoblasts are very variously arranged, scattered, gathered into rows, rings, or on a terminal capitulum: ciliated cells and sense-cells furnished with stiff sense-hairs are present as well. The latter in the Trachynemid Marmanemidae may be aggregated in a patch or a circle at the apices of the tentacles, in longitudinal rows along them, or in 'tactile combs' situated in pairs at the bases of the tentacles as well as between them. The ectodermal musculature of the solid tentacles is striated.
The tentaculocysts are primitively four and interradial, and so persist in Petasus and Dipetasits, but they are usually displaced by the unequal growth of the bell-margin, or the formation of interradial tentacles; there are usually four radial tentaculocysts as well, and then the eight organs take an intermediate or adradial position. The number is seldom increased; e. g. Olindias has one to two hundred or more. Among Geryonidae the Liriopidae have eight, the Carmarinidae twelve, half radial, half interradial, and doubling the number of the radial canals. They are free always at first, and persist in this condition in the family Aglauridae, the Petasid sub-family Petachnidae, and the Trachynemid Pectyllidae, but in the Petasid Olindiadae, the Trachynemid Marmanemidae, and the family Geryonidae they become inclosed by the growth of the ectoderm of the bell-margin in vesicles, which in the last-named are sunk within the exumbrellar mesoglaea. The endodermal axis consists of two to three, rarely four cells, the apical cell dilated, and lodging a single rounded or ovate calcareous otolith.
The ectoderm consists in part of auditory sense-cells. Marginal bulbs and cirri (p. 761) are occasionally present.
The manubrium is very muscular, short when attached to a gastric peduncle (supra), elongate in other instances. The mouth has four lobes, or in the Carmarinidae six, beset with cnidoblasts: it is, as a rule, extremely dilatable, even exceeding the diameter of the bell, e.g. in Liriope, and may then be used as a sucker. There are radial canals, four in Petasidae, as in the Geryonid Liriopidae, eight in others, except the Carmarinidae, where there are six; they are united by a circumferential or marginal canal. In the Petasid Olindias, the Trachynemid Pedis, and in some Liriopidae and Carmarinidae centripetal radial canals originate from the marginal canal between the radial canals, but fail to reach the base of the manubrium. Their number is variable, but increases with age. The sexual organs are developed on the radial canals, in the Aglaaridae alone sometimes at or near the base of the manubrium. They either occupy the whole sub-umbrellar wall of the canals, or are divided by radial muscles into two halves.
They have a narrow base in the Aglauridae, and depend into the bell cavity: and in the Geryonidae they are leaf-like, the apex of the leaf being turned towards the bell-margin 1.
1These tentacles are solid in Pedis and Pectyllis, and arranged in vast numbers along the edge of the bell: some of them are sessile. In Pectanthis they are aggregated in sixteen bundles, two bundles between each pair of radial canals. Pectanthis was observed by Haeckel alive: it was able to climb up the glass sides of an aquarium by means of its suckers, or to anchor itself with the bell-margin downwards or reversed. See his report on 'Deep Sea Medusae,' Challenger Reports, iv. 21, and PI. VIII.
The ovum is naked, as it always is in Craspedote Medusae. The smallest Medusan ovum known is that of Cunina proboscidea ('024 mm.), the largest that of Polyxenia (Solmisstis) albescens (1.5 mm.). Segmentation is regular; in Polyxenia (Solmoneta) fiavescensvar. leucostyla variably irregular; in Aglaura hemistoma irregular. There is a solid morula, or in Geryonidae a blastula. The endoderm in the latter is derived by primary delamination, i.e. transverse fission of the cells: in the former in various ways; (1) by immigration of cells at any point, Aeginopsis (Solntundelld) mediterranea; (2) by secondary delamination, i. e. by arrangement of the cells in two layers, Aglaura hemistoma, the Trachynemid Rhopalonema velatum; or (3) in a mixed mode, primary or secondary delamination, immigration, Polyxenia leucostyla.
In the Narcomedusae the further development of some Solmaridae, and the life history in part of some Cunanthidae have been traced. As to the former, Solmundella s. Aeginopsis mediterranean Solmoneta s. Polyxeniafiavescens, var. leucostyla, have an elongated ciliated free-floating larva - the two ends of which become transformed into the two primary tentacles, the central region acquiring a mouth. In the latter of the two named, two tentacles have been observed developing at right angles to the two first, and a tentaculocyst appearing in each interval between the tentacles. Later stages with eight and twelve tentacles have been seen, but the number of tentaculocysts present was not constant. The central part of the bell is produced by the growth of the aboral part of the body within the circlet of tentacles: the peripheral part by the growth of a ridge in the same zone with the tentacles or to their oral side. But the development of this region has been more accurately traced in some Cunanthids. The lobes are produced by the more rapid growth of the parts between the tentacles; the peronia by the union of the ectoderm fringing the outer surface of the margins of adjacent lobes, whilst the velum appears to be an outgrowth of their inner or oral aspect.