Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/574

558 558 HYDKOZOA D). Four new tentacles, those of the intermediate or secondary radii, now appear between the first four, and are termed interradial. At the same time four longi tudinal ridges grow forward on the wall of the enteric cavity (fig. 26). These interradial ridges have sometimes It is in con- y FIG. 28. Surface view of the sub-umbrella or oral aspect of Aurelia aurita, to show the position of the openings of the sun-genital pits, GP. In the centre is the mouth, with four perradial arms corresponding to its angles (compare fig. 26). The four sub-genital pits are seen to be interradial. x indicates the outline of the roof (aboral limit) of a sub-genital pit ; y, the outline of its floor or oral limit, in which is the opening (compare 6 of fig. 16). been erroneously described as containing each a longitudinal canal connected with a circular canal at the base of the tentacles. They are in reality solid, as is the margin of the hypostome from which the tentacles spring. Ti : ~ - -~ nexion with these four ridges that the gastral filaments will subse quently appear, as also the genital organs either along their middle line or adradially to them. The ridges correspond to the mesenteries of the Anthozoa. Eight additional tentacles placed one on each side of the perradial ten tacles (or of the inter radial, according as we may choose to regard the matter) next appear, and are distinguished as  adradial. All the ten- X tacles reaching an equal size we obtain the ap FIG. 29. Half of the lower surface of Aurelia aurita. The transparent tissues allow the enteric cavities and canals to be seen through them, a, marginal lappets hiding tentaculo cysts; 6, oral arms; i&amp;gt;, axial or gastric portion of the enteric cavity; gv, radiating and ana stomosing canals of the enteric system ; ov, ovaries. The gastral filaments near to these are not drawn. (From Gegenbaur.) pearance seen in fig. 26, when the young scyphi stoma is looked at from above. Looked at from the side, with its wide hypostome and short vertical axis, the scy phistoma differs widely from an ordinary hydra-form, and approaches the medusa-form, to which its f uur longitudinal gastral ridges further assimilate it. The little creature is now about an eighth of an inch in height ; in other genera, but not in Chrysaora, it may now multiply by the produc tion of a few buds from its fixed basal disc. After nourish ing itself for a period, and increasing to four or five times the size just noted, the vertical axis elongates and a series of transverse constrictions appear on the surface, marking off the body of the scyphistoma into a series of discs (figs. 26 and 27), each of which by the development of tentacles arid completion of the constriction will become a separate medusa (in its young state called &quot;ephyra&quot;). The tentacles of the Aurelia and the structure of the margin of its hypostome are very different from those of the scyphistoma. They are exhibited in their earliest condition (when the Aurelia-medvLsa, is first liberated from its attachment and is an ephyra) in fig, 26, E, F. The margin of the hypostome is drawn out into eight arms (which are not to be confused with tentacles) ; the end of each arm is bifid, carrying a pair of lappets the marginal lappets which persist in the adult (see figs. 30 and 31). Be tween the lappets is placed a short and peculiar tentacle, the tentaculocyst or sense-organ. The eight arms of the disc and their tentaculocysts are perradial and interradial. As the organism grows, a set of eight adradial tentacles appear in the notches between the eight arms, but never attain any relatively large size in Aurelia. The asteroid arm-bearing ML FIG. 30. Tentaculocyst and marginal lappets of A urclia aurita. In the left- hand figure ML, marginal lappets; 1 tentaculocyst; A, superior or aboral olfactory pit; MT, marginal tentacles of the disc. The view is from the aboral surface, magnified about 50 diameters. In the right-hand figure A, superior or aboral olfactory pit; B, inferior or adoral olfactory pit; //, bridge between the two marginal lappets forming the hood ; T, tentaculocyst ; End, endoderm ; Ent, canal of the enteric system continued into the tentaculocyst; Cun, endo- dermal concretion (auditory) ; oc, ectodermal pigment (ocellus). The drawing represents a section, taken in a radial vertical plane so as to pass through the long axis of the tentaculocyst. (After Eimer.) character of the margin of the disc is soon obliterated by the relative growth of the intermediate adradial areas, which become quite filled up, so that in the adult the tentaculocyst is carried in a notch instead of on a prominence, and is concealed by the two lappets (figs. 28 and 30). The margin of the disc between adjacent pairs of lappets gives rise to a fold which grows inwards (toward the mouth) during an early stage (fig. 31), and numerous small tentacles (the fringe) appear along the margin of the disc, which soon equal in size the first adradial tentacle. The in- ,., , . , , FIG. 31. Part of the margin of the disc growing fold IS the velum Or of n young Awetia, to show the rudi- &quot; nPlldo vplnm &quot; and nPVPr mentaiy velum, Vet, extending from uuo-vemm, dim nevei the margina i la pp ets, ML, on either increases in size, SO that in side; T, the small tentacles fringing the adult it is not observ able. The tentacles also remain very small and fine in Aurelia, forming a continuous fringe along the edge of the disc, interrupted only by the eight notches for the tentaculocysts (fig. 29), The sixteen tentacles of the scyphistoma are necessarily attached to the most anterior of the pile of medusae ; they atrophy, but to what extent they may be metamorphosed to form the parts of the ephyra or young medusa has not been determined. The scyphistoma, having given rise to its pile of ephyree, may (in some genera, Aureliaf) redevelop its own kind of tentacles belcw the constriction marking off the last ephyra. Hence scyphistoma tentacles appear sometimes at the top and sometimes at the bottom