Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/573

557 HYDROZOA 557 edges of the oral opening fuse together at an early age and leave several sucker-like secondary mouths, which were formerly mistaken for independent persons. The central enteric chamber is continued through the disc by a com plicated often reticulate system of radiating canals, which excavate the endoderm lamella. development have recently formed the subject of investiga tion by Glaus, Eimer, and others. As the current accounts FIG. 24. Scyphomedusoe. a, Rhizostoma pulmo; b, Chrysaora hyoscnua In the Semostomcc and Rhizostomce (not in the Cubostomce) four remarkable (respiratory) sub-genital pits (fig. 28) are hollowed out in the gelatinous substance of the sub-umbrella (oral face of the umbrella). These do not communicate, as FIG. 1o Four stages m the development of Chrysaora. A, Diblastula stage ; B, stage after closure of blasfopore ; C, fixed larva with commencing stomodwum or oral ingrowth ; D, fixed larva with mouth, short tentacles, &c. ; ep, ectoderm hy, endoderm ; st, stomodajum ; m, mouth ; bl, blastopore. &amp;lt; From Balfour, after x/J&US.) has been erroneously supposed, with the genital organs, the products of which normally are evacuated by the mouth. In the Tetragamelian Rhizostomce these pits remain distinct from one another as in Semostomce, but in the Monogamelian Rhizostomce they unite to form one continuous sub-genital cavity placed between the wall of the enteric cavity and the polystoinous oral disc. The common English forms, Aureha, Chrysaora, and Cyancea, are types of the Semo- stoma, the somewhat less common Rhizostoma of the Monogamelian Rhizostomce, whilst Nausithoe and Disco- medusa represent the simple Cuoostomce. The writer has adopted the term used by Haeckel for this order, ana is indebted to his preliminary notices of a large work on the sec, now in the press, for outlines of the classification and de- ions which have been introduced with modifications in relation se and the other Mcdusce. The term Discophora is used by i(Grundziige for the Discomeduscc. It is quite clear from the led and inconsistent use by different authors of that term, and r the terms Acalephce and Mcdusce, that they must be ejected altogether from use in systematic treatises. The structure of the common Aurelia aurita and its in text-books are very inadequate, a short sketch of the morphology of that form is appended here. From the egg, according to the researches of Glaus (whose figures, here repro- duced, refer more especially to the closely allied genus Chrysaora, up to the comple tion of the scyphistoma), a single-cell-layered blastula de velops which forms a diblastula by invagination (fig. 25, A, B, C). The orifice of invagination closes up, and the ciliated &quot; planula &quot; (as this stage used to be termed in all Ccelentera), after swimming around for a time fixes itself, probably by FlG 27 ._ DeveIopmcnt of the blastoporal pole. The true Above to left, young scyphistoma mouth then forms by inruption at the opposite pole. Two ten tacles now grow out near the mouth opposite to one another (fig. 25, D), and are followed by two more (fig. 26), these indicating the four primary radii of the body which pass through the angles of the four- sided mouth, and are termed perradial. Meanwhile the aboral pole narrows and forms a distinct stalk, which in Chrysaora secretes a horny perisarc (fig. 25, with four perradial tentacles. Be low to left, scyphistoma with six teen tentacles and first constiiction. To the right, stiobila condition of the scyphistoma, consisting of thir teen metameric segments; the up permost still possesses the sixteen tentacles of the scyphistoma; the remainder have no tentacles, but are ephyrae, each with eight bifid arms (processes of the disc). Each segment when detached becomes an ephyra, such as that drawn hi tig. 26, E, F. (From Gegenbaur.)