Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/575

559 HYDROZOA 559 of the pile, which has led to diverse accounts of the mode of development of the ephyrae. Whilst changes are going on in the configuration of the margin of the disc of an ephyra on its way to the perfect form of the adult Aurelia, the enteric cavity has also under gone most important changes. Foremost in importance is the development of a single gastral filament on each of the four gastral ridges which necessarily are present in the transverse slice (so to call it) of a scyphistoma, which becomes an ephyra (fig. 26). These rapidly increase in number as the ephyra grows. Further, the enteric cavity at first follows the outline of the ephyra, sending a process into each arm, but then by adhesion of its walls is converted into a four-lobed central chamber, a marginal canal, and an endoderm lamella. A system of canals, the arrangement of which is seen in figs. 20 and 31, subsequently opens out again certain lines and tracts of the conjoined endoderm walls. In the adult Aurelia we find the mouth surrounded by four large arm-like perradial processes (figs. 25 and 29) (not tentacles), and leading through a short manubrium into a flattened four-lobed chamber, the lobes being inter- radial, and having on their oral floor numerous gastral filaments (rich in thread cells) (6 in fig. 16). Each pouch or lobe gives off a canal, which runs towards the circular canal at the margin of the disc, but breaks up into three or four secondary canals on its way. Between the pouches come off eight other &quot;radiating&quot; canals (adradial), which do not branch, but go straight to the circular canal. The oral floor of the concavity of each lobe of the enteric cavity is occupied by a horse-shoe-shaped frill (fig. 29, ov), either testis or ovary (the sexes being in separate indi viduals). The open arms of the horse-shoe are turned towards the centre of the disc, and the folds of the genital frill are so deep as to show themselves on the outer ecto- dermal wall of the disc. Here, however, there is a very remarkable arrangement, which has rarely, if ever, been correctly described and figured in our common Aurelia. The gelatinous substance of the disc is hollowed out on that part of the oral face corresponding to the position of the genital frills, so as to form four separate extensive pits or chambers. Each of these sub-genital pits has in Aurelia a small round opening on the oral face of the disc (fig. 28, GP), but is otherwise entirely closed, having no com munication with the genital tissues, from which it is separated by a delicate layer of ectoderm (6 in fig. 16). The pits probably serve to admit water for respiratory pur poses into close proximity with the genital tissues. The whole enteric surface, including canals, is ciliated, whilst the ectoderm is not ciliated, but provided with groups of nematocysts. The tentaculocyst in the adult Aurelia is relatively an extremely minute body, completely hidden by the two large marginal lappets (fig. 30, T). Above it (that is, on the aboral surface, as the Aurelia swims) is a deep pit (A), Schafer s fovea nervosa superior, sunk in a sort of bridge which connects the two lappets and overhangs the tenta culocyst. A similar pit (the fovea inferior) exists on the oral surface. These have been recognized by Glaus, Eimer, and the Hertwigs as olfactory organs. The tentaculocyst is seen in section in fig. 30 (right-hand figure), which ex hibits its central cavity continuous with the enteric cavity, its ectodermal pigment spot (eye), and its endodermal mass of concretions (auditory organ). The chief muscular mass of Aurelia, except that of the oral arms, is a circular zone on the oral face of the disc. The muscular fibres are not distinct cells, but transversely- striated processes of the epidermic cells (epidermo-muscular cells) (fig. 9). In the &quot;arms &quot;of other medusa?, and pre sumably of Aurelia, the muscular fibre is formed by inde pendent nucleated cells (fig. 9). The nerve-epithelium from the olfactory pits of Aurelia is drawn in fig. 14. Starting from this and from the cells of the tentaculocysts are nerve-iibres, which spread themselves on the surface of the circular muscular zone in the neigh bourhood of the teutaculocysts, and these are connected each and separately with large isolated nerve-ganglion cells (fig. 15). The nerve-fibre is continued beyond the cell, and in some instances has been traced into a broadened ex pansion lying on a muscular fibre (Schafer). The nerve- ganglion cells lie very superficially immediately below the flat epithelium of the body surface and between it and its muscular processes. The ova and spermatozoa of Aurelia develop in the genital frills from endoderm cells in separate individuals. They pass to the exterior through the mouth. Order 3. Conomechisa?, Scyphomedusce with only four tentaculocysts, and these perradial. A broad velum (so- called pseudo-velum) of complete circular form is present, differing from that of the Hydromedusce in the fact that it is penetrated by canals of the enteric system (Charybdcea). The whole umbrella is bell-shaped. The genital organs are four pairs of lamelliform ridges (fig. 22) which are attached to the four narrow interradial septa that divide the large enteric cavity of the umbrella into four perradial gastro- canal pouches. The lamelliform genital glands hang freely in these pouches. At the edge of the umbrella are four interradial lappet-like prolongations of the gelatinous sub stance of the disc, which support each a long tentacle (fig. 20). The nerve-ring is complete, like that of the Hydro- medusae. There is now no doubt that Charybdcea, which has been placed in each of the two large divisions of the Hydrozoa, must be classed with the Scyphomcdusce. The recent investigations of Glaus (Arleitcn aus dem Zool. Institut zu Wicn, Bd. i. Hft. ii., 1878), as well as those of Haeckel and Fritz Miiller, lead to this conclusion. The term Conomcdusce is adopted from Haeckel, who places here, besides Charybdcva and Tamoya, other forms, a fuller description of which may be expected in his forthcoming System der Medusen. In many respects its quadrangular form, its marginal lappets, its broad enteric pouches in place of fine canals, its vascular velum, and its highly complicated tentaculocysts (fig. 13, B) Charybdcea is peculiar. The simplicity of the enteric system and the arrange ment of the genital glands brings it near to Luccruaria. The ex istence of four interradial groups of gastral filaments, and the dis position of the paired genital glands at the sides of the interradial septa, determine its position to be among the Scyphomcdusce. Its development is not known. Figs. 20 to 23 illustrate the structure of Charybdcea.- Order 4. Peromedusce, Scyphomedusce with four inter radial tentaculocysts. The enteric system consists of three divisions, an aboral main stomach with four interradial gastral ridges and filament groups; a mid-stomach, which communicates by means of four perradial slits with a very large ring-sinus (occuping two-thirds of the umbrella) ; and thirdly, an oral portion or pharynx, with four wide per radial pouches. The genital organs are four pairs of sausage -shaped interradial ridges lying on the oral floor of the ring-sinus. This is a new group founded by Haeckel, of which we have at present no further details. Sub-class II. HYDROMEDUS.-E. These are Ilydrozoa devoid of gastral filaments ; the sexual persons are always medusi- form, the genital glands are developed sometimes from ecto dermal cells, sometimes from endoderm, and are always per radial (in the radii of the first order). The medusiform per sons always possess a muscular non-vascular velum (hence Craspedota] and a complete nerve-ring (hence Cycloneura of Eimer). The marginal sense organs are either ocelli or oto- cysts or tentaculocysts. The diblastula, in all cases as yet observed, is formed by delamination (Balfour). The sexual medusiform persons may develop directly from the egg, but more usually the egg gives rise to a hydriform person the hydroid which differs from a scyphistoma in its elongate