Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/566

550 550 HYDROZOA however, occur on the ectodermal cells, and the solid ten tacles are essentially tactile organs. Placed in and between the large cells of the ecto derm (Hydra, Cordylophora, Allman, Kleinenberg, F. E. Schulze) are small nucleated cells which become con verted into vesicles contain ing a three-barbed (figs. 4 and 5) or simple filament (nematocysts). These are frequently grouped on the surface in wart-like pro cesses or &quot; batteries.&quot; Ne matocysts also are found in the endoderm; but it is prob able that their presence there is due to their having been swallowed. The endoderm is usually but one cell deep, and lines FIG. 5. Nematocyst of Hydra, showing the entire cavity of the body gftg! ^SfS^S^SSi starting from the margin of F. E. scimize.) the mouth. In the region of the body proper, and in hollow tentacles, the cells are ciliated (fig. 4). In this region they are concerned in the secretion of digestive fluids and in absorp tion, and sometimes contain coloured granules (hepatic]). All- man found in Myriothela (Phil. Trans. , 1875) that the endo derm cells project processes like the pseudopodia of Pro tozoa, and suggests that solid food particles are incepted by them. T. J. Parker has published similar observa tion on Hydra (1880). In the solid tentacles the en- dodermal cells are greatly modified, forming a kind of skeletal tissue, each cell recalling by its vacuolation and firm cell-wall the characters of vegetable parenchyma (fig. 6). In the stems of Siphonophora endoderm cells give origin to muscular processes like those of the ectoderm (Glaus). This latter fact has a morphological significance which cannot be too gravely estimated. Generative products are not developed by any hydriform persons (excepting the Litcernarice), the sexual process being carried on by a distinct set of buds developed on the sides of hydriform persons. These buds either become medusi- form persons, or are degenerated representatives of such persons (sporosacs) (figs. 17 and 18). Even the fresh- water Hydra (fig. 42) does not appear to be an exception to this generalization. The single egg cell of Hydra projects at the breeding season in an ectodermal covering, as a wart, from the lower part of the body. A conical eminence or two nearer the mouth contains the spermatozoa. Euch ovarium and each spermarium represents an aborted gene rative person. According to Kleinenberg the egg-cell and the sperm-cells are both derived from the ectoderm. The Lucer iiar ice develop internal generative organs (fig. 19) which correspond closely with those of the medusiform persons of the group Scyphomedusce (see below), with which they are classified. Both ova and testis are endoderm al in origin in Lucemaria and in the medusiform persons of the Scyphomedusce, whilst they appear to be ectodermal in origin in the complete medusiform persons of Hydro- medusa, though in the degenerate medusiform persons known as sporosacs they may either or both have an endodermal origin. MEDUSIFORM PERSONS usually present themselves as isolated free-swimming individuals, but like hydriform FIG. 6. Vacuolatod cndoderm cells of carti laginous consistence from the axis of the tentacle of a Medusa (Ciinina). (From Gegenbuur s Elements of Comparative Anatomy.) persons they have the power of producing new persons by budding (figs. 44, 45, and 46), which may become detached or may remain connected with the primary person (fig. 57) to form a freely swimming colony (Siphonophora) compar able to the fixed colonies of hydriform persons. Medusi form persons are often produced as the immediate result of the development of the diblastula without any intermediate hydriform phase (Pelagia among Scyphomedvisce, Tracho- medusce, Narcomedu8ce,&Q.d probably someAnthomeditsa? and Leptomedusce), but quite as frequently originate as lateral buds upon the body-walls of hydriform persons (figs. 34, 37, and 43), or of other medusiform persons (see below), or as metameric fission-products of hydra -forms. The typical medusa-form is ,a hemispherical cup (the nectocalyx, or umbrella, or disc), from the centre of which rises up a cylindrical or conical process (the manubrium, erroneously polypite) at the summit of which is the mouth (4, 5 in fig. 1G). Four perradial (see above for use of this term) ten tacle-like lobes very commonly surround the mouth, or numerous small tentacles (fig. 58), whilst the margin of the disc is beset with tentacles four in number, or a mul tiple of four (sometimes six, or one only, or indefinite). The aboral pole is dome-like, and is never attached except in those forms which take their origin as buds on a hydri form colony when the connexion exists at this point. The tentacles are, as in the hydriform persons, some solid, some hollow ; both occur in the same individual. FIG. 7. Portions of sections through the disc of medusa;, the upper one oiLizzia, the lower of Aurelia. el, endoderm lamella, or vascular lamella; m, muscular processes of the ectoderm cells in cross section ; &amp;lt;/, ectoderm ; en, endoderm lining the enteric cavity; e, wandering endoderm cells of the gelatinous sub stance. (After Hertwig.) The body is not so completely hollowed out as in the hydriform persons. The mouth leads into a straight tube (the stomach) which occupies the axis of the manubrium, and expands at its insertion into the disc. The disc, even when thick and fleshy, is not fully excavated by the enteric cavity. In young forms the cavity does occupy it right up to the margin, but gradually the lumen disappears (fig. 29), leaving a series of canals and a continuous plate of endo derm (fig. 7) formed by the coalesced walls of the space (the endoderm-lamella of the Hertwigs, see Orc/anismus der Medusen, 1878; the vascular-lamella of Glaus, &quot; Polypen und Quallen der Adria,&quot; Wiener Deiihch., 1878). The peripheral portion of the lumen of the original enteric cavity forms the ring-canal, which runs all round the margin of the disc, and is continued into the hollow tentacles. The lumen is further retained at intervals in the form of radiat ing canals connecting the axial enteric cavity with the ring- canal. These may be perradial, interradial, and adradial (see above as to tentacles of hydra-form), and may branch dichotomously in the disc or form networks. The medusae are thicker and more fleshy to the touch than are the hydra-forms, and are at the same time trans parent. This is entirely due to the enormous development of a structureless substance between ectoderm and endoderm, corresponding to the &quot; Stutz-lamella&quot; or structureless lamella of the hydra-forms. (See figs. 49 and 51, representing sections of Carmarina and of Cunina.)