The characters above given apply to the second division of Acraspeda, the Ephyroniae of Haeckel. A large number of Medusae belong to it, and it contains three orders, the Cannostomae, Semostomae, and Rhizostomae.
The first division of Acraspeda, the Tesseroniae, contains relatively-few forms. The bell is of great depth; and in the Depastridae and Lncernaridae it is produced into a hollow peduncle by which the animal attaches itself at will. Its cavity is prolonged aborally into four interradial 'funnel cavities' of variable extent, corresponding to the four sides of the manubrium. The angles of the latter are consequently attached by four perradial septa, ' mesogonia,' to the subumbrellar wall. Rhopalia are absent in the Stanromednsae and their place may be taken in Lucemaridae by adhesive tentacles, sometimes lost, or by simple tentacles in Tes-seridae. They are four in number when present, interradial in Pero-medusae, perradial in Cubomedusae. They bear one or two dorsal eyes in the former, sometimes also a ventral: in the Cubomedusan Charybdaea marsupialis three pairs, a large central and two lateral pairs. In the last-named the retinal layer is cup-shaped; the cup filled by a vitreous body, to which is added in each of the large eyes a lens formed by elongate ectoderm cells much as in Vertebrata. Charybdaea has also a continuous nerve-ring, in position corresponding to the inner nerve ring of Craspedota. It consists of fibres and ganglion cells, the latter especially aggregated at the bases of the rhopalia, with an overlying ectoderm of sense and supporting cells.
Tentacles are usually limited in number, and are rarely solid. In the fixed forms they are capitate, ranged round the bell-margin in Depastridae, grouped at the end of eight short hollow adradial arms in Lucemaridae. The mouth is square. The main gastric cavity consists of four pouches communicating peripherally by apertures of varying size, depending on the length of the septa that separate them 1. Processes from the cavity pass into the marginal lobes of Peromedusae, and the velarium, when present, of Ctibomedusae, e.g. in Charybdaea. The taeniolae are greatly developed in most Stauro- and Peromedusae, extending along the ex-umbrellar wall of the gastric cavity to a greater or less extent. The funnel cavities penetrate them in some Lucernaridae and the Peromedusae. The gastral filaments may be only four, as in Tessera, but are generally, especially in the Peromedusae, extremely numerous, and in Cttbomedusae aggregated into a single or double group at the axial ends of the gastric septa. In Stauro- and Pero-medusae they frequently extend along the exumbrellar wall of the gastric cavity even to its centre.
The genital organs are situated on the subumbrellar walls, but in one section of Lucernaridaey the Halicyathidae or Cleistocarpidae, are contained in special mesogonial or gastrogenital pouches opening into the central gastric cavity and placed perradially in the cavity of the bell. The organs are either horseshoe shaped, the convexity of the curve being adcentral, or the two limbs of the horseshoe are separate (Lucernaridae, Peromedusae). In the Cubomedusae there are eight genital lamellae, attached two to each gastral septum, one on either side 1.
1 Claus points out (Untersuchungen, p. 14) that the starting-point of the Tesseroniae is conceivably the stage in the formation of the Ephyra from the Scyphostoma at which the taeniolae are perforated by passages putting the four gastric pouches into communication. The gastric cavity becomes much complicated in the Peromedusae: see Haeckel, System, p. 402 et seqq. There seems to be some confusion as to the character of the septa dividing the gastric pouches. In Charybdaea marsupialis each septum is traversed by a layer of endoderm cells, showing that there has been a fusion of the two walls of the gastric cavity. On the contrary, the septa of Lucernaridae, as may be seen from Claus' figures (Untersuchungen, Pl. IX. Fig. 62; PI. X. Figs. 71, 72), contain no such layer, and are probably taeniolar growths. The distinction is certainly one of importance. Haeckel appears to think that the septa in question, whether nodes, lines or lamellae, are always formed in the same way: see Deep-sea Medusae, Challenger Reports, iv. pp.lxix.-lxx.
The Depastridae and Lucernaridae are probably to be regarded as specialised Scyphostomae, and as standing apart from other Acraspeda.
The development of the Tesseroniae is unknown save to a certain extent in a Lucernaria. In it segmentation is equal and results in the formation of a morula. The endoderm is formed either by delamination from the ectoderm cells at one pole or by an immigration (?) of ectoderm cells at all points into the centre of the morula (Gotte). The larva is non-ciliated (? in all cases) and creeps about; it is elongated; one pole is beset with cnidoblasts, by the other it fixes itself2. The ovum of Ephyroniae is either set free from the ovary and has then a vitelline membrane, or in Nausithoe marginata a mucous coat with cnidoblasts, or, as in Chrysaora, remains enclosed in an envelope of follicle cells. In the first case it may pass into the planula condition while attached to the oral arms of the female (Aurelia, Cassiopeia), and it may even nearly attain the Scyphostoma stage in this position (Cyanea Annaskala, Stylorhiza punctata), or the embryoes are contained in pouches of the radial canals (Pseudorhiza aurosd). Segmentation is regular: the blastocoele is large in Pelagia and Chrysaora, small in other cases.
The endoderm is formed by an invagination the cavity of which is linear: the gastrula mouth closes but the pole which corresponds to it is denoted by the development of cnidoblasts, and is the one at which the mouth of the adult is formed later on. The planula ( = two-layered blastula) is sometimes ciliated and free-swimming (Atirelia, Chrysaora). When it fixes itself the mouth is formed: two perradial tentacles first make their appearance opposite to one another, then the remaining two, finally the interradial tentacles and taeniolae simultaneously. The planula of Pelagia developes direct into an Ephyral.