(Cryptocarpa, Gymnophthalmata, Hydromedusae).
Hydroid form, either a free and temporary larval stage, or permanent, and then either free or fixed, solitary or coloiiial either temporarily or permanently. It may or may not be tentacidate; its tentacles rarely hollow, usually solid; its mouth prominent and gastric cavity simple. It is sometimes polymorphic. The skeleton is either a chitinoidperisarc, or more rarely a calcareous coenosteum. Asexual reproduction takes place very rarely by fission, generally by gemmation; and in the permanent hydroid forms some only of the buds become sexual zooids.
The Medusa has a tubular manubrium, the margin of the bell even or lobed, and provided with an initurned velum, a double nerve ring (inner and outer), and sensory organs either ocelli or auditory organs; the latter either tentaculocysts or ectodermic otocysts. It may become sessile and degenerate to a greater or less extent. The sexes are separate; the sexual cells typically of ectodermic origin, but sometimes, especially in the degenerate forms of sexual zooid and the Siphonophora, formed in the endoderm. Marine with few exceptions.
There are three orders, Trachymedusae, the Hydroidea, and Siphonophora.
The order Trachymedusae, the Trachylinae or second sub-legion of Craspedota of Haeckel, contains Medusae which possess tentacles with a solid axis, sometimes replaced in the adult by hollow tentacles, and ten-taculocysts or auditory tentacles, with an axis of endodermal otolith cells. Ocelli are rare. Development is direct by metamorphosis from free hydroid larvae, and there is an Alternation of Generations only in one parasitic species. 'Sporogony,' or development from a non-sexual spore, occurs in a few instances. There are two sub-orders, the Narcomedusae and Tracho-medusae.
In the Narcomedusae, the exumbrella is flattish, rarely bell-like, and is divided into a central and peripheral part by the insertion of the tentacles. Its substance is firm and traversed by radial, branched and anastomosing fibres. The peripheral portion is divided into lobes, one lobe between every two tentacles. The lobes are connected together by the subumbrella and by well-marked intervening streaks or 'peronia/ of ectoderm containing cnidoblasts, continuous with a marginal band of cnidoblasts and ciliated sense-cells, and reaching from the latter to the bases of the tentacles. A nerve extends from the outer ring beneath each peronium. In the Pegan-thidae, however, the subumbrella is also divided, the peronia are very rudimentary, and the lobes are united by the velum. The subumbrella corresponds solely to the peripheral region of the exumbrella, the central portion being occupied by the stomach. The velum is broad; it is either horizontal or hanging vertically downwards. In locomotion the lobes of the bell are turned inwards with the velum.
The tentacles may be in number only two, or four, or eight, but in the majority more numerous, but never more than thirty-two. In the Aeginidae their number follows a geometrical progression, but in the other three families it is irregular. Their ectoderm consists of cnidoblasts and sensory cells, some ciliated, some provided with stiff sense-hairs. The axial endoderm cells are developed in a single row, but may be numerous at the base, and are continued as the tentacle-root for some distance along the ex-umbrellar aspect of the stomach. The tentaculocysts may be only four in number, alternating with the four tentacles in Cunantha, generally more numerous, often several hundreds in number. They are inserted on the marginal band of the lobes (supra). The auditory ectoderm cells have long auditory hairs: the axial endoderm cells are in a single row, as a rule two to four, rarely more in number. Of these one, or two to four, contain one or more calcareous otoliths, as a rule of regular crystalline figure, rarely, as in the Solmaridae, globular.
The tentaculocyst is frequently borne upon an auditory ectodermal papilla composed of sense-cells with long auditory hairs. 'Otoporpae,' or meridional streaks of ectodermal cnidoblasts extending centripetally on the exumbrella from the bases of the tentaculocysts are found in the Cunanthidae and Peganthidae.
A prominent manubrium is present in some Cunanthidae and in many Aeginidae,but not in other instances. In the last-named the mouth is generally four lobed: it is always extremely dilatable. The central stomach cavity is usually flattened. In the Cunanthidae it is produced into radial pouches, a pouch corresponding to each tentacle. Every pouch is connected to the pouch on either side by a separate 'festoon' or loop-canal, which follows the edge of the corresponding lobe. The two festoon canals originating from each pouch are separated at their roots by a peronium. The Peganthidae have a central stomach and festoon canals derived from it corresponding to each lobe: the Aeginidae a variable number of interradial pouches extending into each lobe, and festoon canals derived from the central stomach as well. In the Solmaridae the festoon canals are obliterated: the stomach may be simple and central, or prolonged into radial or interradial pouches.
The sexual organs are developed on the subumbrellar aspect of the stomach, either as a simple ring which may extend in some Cunanthidae either on to the radial pouches, or become restricted to them. The ring may be resolved in some Peganthidae into a number of separate interradial pouches, as it always is in Aeginidae. In Solmaridae all three conditions may obtain.
Most Trachoniedusae have a firm, stiff, semi-globular, or broad bell. In the Aglauridae, however, it is bell-shaped, cylindrical, or eight-sided and prismatic; and the exumbrella is excessively thin. The velum is broad and stout, and hangs downwards when the Medusa is floating. The margin of the exumbrella has a strongly developed ring of cnidoblasts, and where the solid tentacles are sub-marginal, as in all Geryonidae and some Peta-sidae, there are peronia, beneath which the roots of the solid tentacles, when cast off, persist in connection with the circumferential canal. The exumbrella is prolonged in the Aglauridae and Geryonidae into a long 'gastric peduncle/ - a solid cylindrical process of its gastric aspect, which is in the last-named usually longer than the height of the bell, and therefore projects from the bell-cavity. Mesogonia occur in the Pectyllidae: see p. 786.