Hydroid form known in a few instances only; small and fixed, protected partially in some instances by a perisarcal tube; with a peristomial disc bordered by sixteen (or more) solid tentacles; mouth squarish and gastric cavity traversed by four vertical interradial ridges; multiplying either by lateral buds which are detached, or by creeping stolons which throw up buds at intervals. The Medusa is derived from it by multiple transverse fission of the oral extremity of the body, giving rise to a temporary form known as Strobila.

The Medusa has a square-shaped manubrium, the oral angles of which are sometimes produced into four simple or branched arms; bifid marginal lobes to the bell rarely absent, either four perradial or four perradial and four. interradial, supporting as many sensory bodies, i.e. tentaculocysts, or rhopalia; more or fewer gastral filaments disposed in rows or groups (phacelli) within the central gastric cavity at the sides of the manubrium; and genital organs similarly placed with a genital epithelium of endodermal origin. The sexes are separate: development is direct in Pelagia. There is no velum to the bell. Tentacles may be absent, or, if present, either situated on the exumbrella or subumbrella. The nervous centres are contained in the marginal sensory bodies. Marine.

The non-sexual Hydroid form of the Acraspeda, the Scyphostoma or Hydra tuba, is only known in genera of the Ephyroniae or second division of the sub-class2. It is relatively small in size (half-an-inch or less), and its stoma Cuvieri, Pseudorhiza aurosa, Stylorhiza punctata. As these genera represent several families, there can be little doubt that the Scyphostoma is of general occurrence.

1See p. 747, ante, for Gotte's view as to the position of this sub-class.

2That is to say in Nausithoe marginata, Chrysaora (C. isosceles, C. Mediterraned), Cyanea (C. arctica, C. capillata, C. Annaskald), Aurelia (A. aurita, A.Jlaviduld), Pilema pulmo= Rhizospecial characters are as follows. The mouth is dilatable and somewhat squarish in outline, especially in early stages: its margin is thickened and armed with cnidoblasts: 'it is situate in the centre of a disc or peristome of great mobility. The margin of the peristome carries normally sixteen tentacles: the number may however be greater, e. g. twenty or thirty-two. The tentacles are contractile, and have a solid core of endoderm cells. Four of them, the pet-radial tentacles, which are first developed, correspond to the four angles of the mouth; four others, the interradial tentacles, second in development, to the centres of the square sides of the mouth, and the remaining eight adradial tentacles occupy the intervals between the per- and inter-radial. The body is somewhat elongated, but contracts below into a peduncle, by which the animal is affixed to some foreign object (stones, sea-weeds, &c). This peduncle is slender in Chrysaora and Cyanea, and inclosed in a gelatinoid sheath secreted by the ectoderm1. The young Nausithoe marginata inhabits a perisarcal tube.

The internal aspect of the gastric cavity is traversed by four vertical ridges, septa or taeniolae, which correspond one to each of the four interradial tentacles. They are projecting ridges of mesoglaea covered by endoderm cells and inclosing a tube of muscle cells of endodermal (Claus) or ectodermal and peristomial (Gotte) origin. They have been compared to the mesenteries of Anthozoa. The tentacles especially are provided with nematocysts of two sizes, and the smaller (microcnidae) have extremely long cnidocils. Cilia occur upon the tentacles, in greatest numbers near their bases, as well as upon the peristome and margins of the body. The Scyphostoma multiplies in two ways, by means of lateral buds which are detached, and by two to three creeping basal stolons, from which buds are thrown up at intervals2.

1L. Agassiz states that the sheath is sometimes wanting in Cyanea, and the young animal may be seen creeping on its tentacles.

2F. E. Schulze has described (A. M. A. xiii. 1877), under the name Spongicola fistularis, a Hydroid which he observed inhabiting the canal-system of several marine sponges, e. g. Reniera fibulata, Esperia Bauriana, etc. It forms a colony of nearly vertical branching tubes invested by a. chitinoid perisarc. The Hydroid itself has the typical structure of the Acalephan Hydroid as given above. Its peristome measures about 1-11/2| mm. in diameter. It has 16, 20, etc. up to 40 solid contractile tentacles, and the taeniolae extend across the peristome to the margin of the mouth. The peristome is bordered by a strong circular fold. The exposed part of the body and the tentacles are ciliated. The fore-part of the body bearing the tentacles can be invaginated. Allman has also described a sponge-inhabiting Hydroid, Stephanoscyphus mirabilis, which closely resembles Spongicola. It is colonial, has a perisarcal tube, etc. But its great peculiarity, according to its discoverer, is the presence of a circular and four longitudinal canals. It is probable that the structures in question are really four taeniolae and a circular fold such as are described above.

See Allman, Tr. L. S. (2), i. 1879. Schulze's Hydroid is probably identical with the structures described by Eimer (Tageblatt der Natf. Versaml. in Leipzig, 1872, p. 62), and supposed by him to be integral parts of the sponge in which they occur.

Kowalewsky, quoted by Metschnikoff (Embryol. Studien an Medusen, 1886, p. 88) appears to have seen the strobila of Spongicola and the detachment of Ephyrae.

The Scyphostoma passes in late autumn into the Strobila stage. A circular furrow constricts the body on the aboral side of the tentacular circle, cutting off a disc. A series of similar furrows appear one after another, a basal pyriform portion of the body alone remaining undivided. The disclike segments thus cut off increase in diameter and are detached as young Medusae or Ephyrae (=Ephyrulae, Haeckel). In the first-formed segment eight bifid marginal lobes grow out, each of which embraces the base of a per- or an inter-radial tentaclel. These tentacles shorten, and their basal portions are converted into the sensory bodies or rhopalia of the Medusa. The eight adradial tentacles undergo complete atrophy. The gastric cavity grows peripherally as four pouches separated by the four taeniolae, and a perforation then takes place through the base of attachment of each taeniola putting the pouches in communication. The gastric cavity extends outwards into each of the eight bifid lobes, which are much elongated. The portions of the four taeniolae cut off are usually converted into gastral filaments, and the taeniolar muscles are lost.