Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/567

551 The remarkable development of this substance in a hyaline con dition has led to the description of canals and spaces where none exist the supposed spaces being really occupied by this hyaline substance. F. E. Sehulze s statements as to extra-enteric spaces in Sarsia are thus explained and more decidedly the supposed circular and longitudinal canals attributed by some authors to the scyphi- stoma phase of Discomcdusce. In the same manner (according to Claus) Allman s observations on StcpJutnoscyphus are reconciled with those of F. E. Schulze on Spongicola clearly the same form. Stephanoscyphus is devoid of either circular or longitudinal canals, and though it has four remarkable ridges on the enteric wall like those of the scyphistoma of Scyphomedusae ^see fig. 26) stands in all probability very close indeed to the Tubularian genus, Perigonimus. In a large number of medusa-forms the hyaline gelatinous substance is structureless, but in many of the larger Scy- pkomedusce it is occupied by in-wandering amoeboid cells de rived from the endoderm. and by fibrous trabecula? (fig. 8). Fio. 8. Gelatinous substance of the disc of AureJia, showing a, fibrous tra- beculae, and 6, wandering endoderm cells, with amoeboid movements (From Gegenbaar.) The wandering endodermal cells are nutrient in function, and represent so far isolated elements of the enteric canal system. The ^medusiform person is fundamentally adapted to swimming movements. The muscular fibres are mostly transversely striated, and are as a rule outgrowths of super- FIG. 9. Muscular cells of medusae (lAzzia). The uppermost is a purely muscular 11 from the sub-umbrella; the two lower are epidermo-muscular cells from the base of a tentacle; the upstanding nucleated portion forms part of the epidermal mosaic on the free surface of the body. (After Hertwig.) ficial ectoderm cells as in Hydra (fig. 9), (though in some cases distinct cells) ; they are confined to a sheet spread on the oral face only of the disc or swimming-bell (sometimes called sub-umbrella), to the extensile manubrium and tentacles, and to an inwardly directed flap of the margin of the disc known as the velum (Ve in 4 of fig. 16), which is present in those medusae that are not flattened but conical (bell-like). The muscular fibres on the oral face of the disc and on the velum have a circular direction, interrupted income cases by radial tracts. The direction of the swim ming movements is obvious from this arrangement. _ The velum is not a constant element in the medusa s disc; it serves to contract the space by which water is oxpelled from beneath the bell in the act of swimming. 551 All fully-developed Hydromedusce possess the velum, but only a few of the Scyphomedusce (Charybdva). In the former the endoderm plate (vascular lamella) is not con tinued into it; in the latter vessels of the enteric system are present in it (fig. 21), and, being probably morphologically distinct, it has been here termed the &quot; pseudo-velum.&quot; Unlike the hydra-forms, the medusa-forms of Hydrozoa possess in addition to the tentacles highly-developed sense- organs and gangliouic nerve-centres and nerves. The sense- organs appear to be either eye-spots, or else otocysts, or to combine the functions of both. In addition to these are olfactory tracts or pits connected with the preceding. The sense-organs are placed along the margin of the disc (hence called marginal bodies), and are of three kinds: (1) ocelli rounded pigment spots, rarely provided with a Fig. 10. FIG. 10. Ocellus of a medusa (JLizzia Koellikeri). oc, pigmented ectodermal cells; I, lens. (After Hertwig.) FIG. 11. Otocyst (formed entirely by ectoderm) of Phialidium, one of the vesiculate medusa;, d 1, superficial layer of ectoderm ; d 2 , deep layer of ecto derm; h, auditory cells of ectoderm; hh, auditory hairs; np, nerve body; nr 1, upper nerve-ring; r, endoderm cells of the circular canal. The otolith cavity is seen above h. lens (Lizzia) (fig. 10), always placed at the base of a tentacle or in the radius of one on the oral surface (Lizzia), entirely ectodermal in origin ; (2) vesiculi or otocysts formed (as discovered by the Hertwigs, 1878) by an invagination of the ectoderm (fig. 11) containing concretions and hair cells; either open or entirely closed, generally numerous, and placed between tentacles, sometimes at the bases of tentacles (Obelia) ; (3) tentaculocysts which are reduced and modi fied tentacles; into them alone of the three kinds of mar ginal bodies do the endoderm and, in the more complex, the enteric canal system enter (figs. 12, 13, and 30). The endodermal sac forms the axis of the tentaculocyst, its cells secrete crystalline concretions, and it functions as an otocyst; pigment spots, which may have cornea, lens, and retina well developed, are formed sometimes to the number of six (Charylda a) on the ectoderm of the tentaculocyst (fig. 13). The olfactory sense-epithelium (fig. 14) is either dis tributed in a continuous band on the margin of the disc (Hydromedutoe, discovered here by the Hertwigs), or it La