Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/578

562 562 HYDROZOA have no otocysts, but only ocelli {Ocdlatci;}, or they have otocysts (fig. 11) (ectodermal sacs), four, eight, or over a hundred, not homologous with tentacles, and sometimes in addition ocelli ( Vesiculatce). The radial enteric canals are usually four or eight in number, but may be more numerous, whilst the marginal tentacles of the disc are either few or FIG. 45. Medusiform person (Sarsia), one of the AnthomeduHf, detached from a hydroid colony of the family Corynidce. b, the long manubrium, tearing (as an exception) medusiform buds; a, month. FIG. 46. Medusiform person, one of the Anthomedusce, detached from a hydro... colony of Syncoryne. Ocelli are seen at the base of the tentacles, and also (as an exception) groups of mednsiform buds. very numerous. The genital glands always are placed in the course of the radial canals of the disc (not in the manu brium), and stand out as groups of wart-like processes on the sub-umbrellar surface (fig. 43). Their mode of dis- . charge is uncertain. FIG. 47. View of the oral surface of one of the Lcptomcdusoe (Irene pellucida, Haeckel), to show the numerous tentacles and the otocysts. ge, genital glands ; M, manubrium; ot, otocysts; re, the four radiating canals; Ve, the velum. The Calyptoblastea of All man, Sfcenotoka, of Cams, and Campanu- laricc of authors form a well-marked group of hydroids which, when they give rise to free meduste, give rise to those termed Leptomedusce by Haeckel, corresponding to the Thaumantiadce and Uucopidce of Gegenbaur s system. The calyptoblastic hydroid Leptoscyplms, which, according to Allman, gives rise to a Lizzia-like medusa (Antho- medusce), is the only recorded exception to this correspondence. The sEquoridce and other medusa of similar structure have not been traced into connexion with any hydriform trophosome, but we are not justified therefore in concluding that they develop directly from the egg without hydriform phase. The chief point distinguishing the Lcptomedusce as a lot from the Anthomeduscc is the development of the generative bodies in the radial canals. This position is simi lar to that occupied by the same organs in Trachomcdusce and Scyphomcdusce. Allman, however, considers the genital glands of the Leptomedusce, not as mere glands like those of Aurdia or Charybdcea, but as a series of buds a generation of aborted medusae or sporosacs. In consequence he terms the medusa of the Leptomedusce a blastocheme (or bud-producer), as distinguished from a gonocheme (or genital-producer). In support of this view, Allman (Monograph, 1874) adduces the various remarkable cases of production of buds by medusse which have been recorded (fig. 44, 45, 46), and, further, the very striking similarity between the structure of a lobe of the genital gland of Obelia and a sporosae such as we find in If ydr actinia. It seems necessary to accept Allman s view on this matter, unless we are prepared to abandon the homology of sporosacs with medusae in the case of hydriform persons. The colonies of hydriform persons of the present group differ inter se according to the arrangement of the cups or hydrothecse. In Plumularidce they are sessile, and all on one side of a branch ; in Scrtularidcc they are sessile, and alternately placed on either side ; in Campanularidce each cup is raised on a pedicel or stalk. The medusiform persons sometimes remain abortive and sessile in their gonangia. - h FIG. 48. Carmarina(Geryonia) hastata, one of theTrachcmediisce. (After Hacckcl.) ,nerve-ring; a, radial nerve ; b, tentaculocyst; e, circular canal ; e, radiating canal; g&quot;, ovary ; h, peronia or cartilaginous process ascending from the carti laginous margin of the disc centripetally in the outer surface of the jelly-like disc; six of these are perradial, six interradial, corresponding to the twelve solid larval tentacles, resembling those of Ct;nina; t, dilatation (stomach) of the manubrium; J, jelly of the disc; p, manubrium; t, tentacle (hollow and tertiary, i.e., precede d by six perradial and six interradial solid larval tentacles) ; , cartilaginous margin of the disc covered by thread-cells ; r, velum. Order 3. Trachomedusce,- sense-organs tentaculocysts. -Hydromedusw which have as The otoliths (fig. 12) are Fio. 49. Diagram of a vertical section of Cannai-ina hastata, passing on the right through the whole length of a radiating canal, and on the left through the outspread lobe of an ovary. I, gelatinous substance of the disc and gastric stalk (manubrium); r, radiating canal; rs, its outer, rl, its inner wall; g, ovaries; k, stomach (dilatation of the manubrium); Z, tongue-like process of the gelatinous substance ; h, cartilaginous process ascending from the marginal ring at the site of a tentaculocyst; c, circular canal; b, tentaculocyst; i , velum; uk, cartilaginous marginal ring. (From Gegcnbaur.) formed by endodermic cells as in /Scyphomedusce, and ocelli may or may not be present on the tentaculocyst.