Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/572

556 556 HYDROZOA diblastula in all cases, as yet observed, is formed by in- vagination, the blastopore closing up (Balfour). Fig- 23. FIG. 20. Charybdcea martupialit (natural size, after Onus). The four annulated tentacles are seen defending from the four lappets placed at the four corners of the quadrangular umbrella. These are interradial. Two of the four perradial enteric pouches of the umbrella, representing radiating canals, are seen of a pale tint. Fg, gastral filaments (interradial); R, the modified perradial tentacles forming tentnculoeysts; G, corner ridge facing the observer and dividing adj-icent pouches of the umbrella; GF, position of one of the genital bands. FK;. 21. View of the margin of the umbrella of Ckarybthta marsupiatis (natural s ; ze, after Glaus). At the four corners are seen the lappets which support the long tentacles, and in the middle of each of the four sides is seen a tentaculo- cyst. Vel, the vascular velum or pseudo-velum, with its branched vessels. FIG. 22. Horizontal section through the umbrella and manubrium of C harybtlcea marsupialis (modified from Claus). Ma, manubrium; SR, side ridge &amp;gt;en-adiul); CR. corner ridges, separated by CG, the interradial corner groove; Ge, tile genital lamellie in section, projecting from the interradi;il angles on each side into VE, the enteric pouches of tin- umbrella; SU, the sub- umbrella space. FIG. 23. Vertical sections of Chartibdiea marsu/tia/if, to the left in the plane of an interradius, to the right in the plane of a perradius. Ma, manubrium; EAx. axial entcron ; Gh, gastral filaments (fifiacellce); CG, corner groove; SR, side ridge ; EiiL. endoderm lamell i (line of concrescence of the walls of the enteric cavity of the umbrella, whereby its single chamber is broken up into four pouches); Ge, line of attachment of a genital band; F.U, enteric pouch of the umbrella, in the left-hand figure, points to the cavity uniting neighbouring pouches near the margin of the umbrella and giving origin to TCct, the tentacular canal; Ve, velum; Fr, freuum of the velum; Tc, tentaculocyst. The binary division of the Hydrozoa was established by Esch- scholtz (System der Acalephcn, 1829) whose Discophorce. phanero- carp(K correspond to the Scyphomcduscc, whilst his Discophorce cri/ptocarpce represent the Hydromcdtisne. The terms point to dis tinctions which are not valid. In 1853 Kulliker used the tcTmDis- coplwra for the Scyphomcdusce alone, an illegitimate limitation of the term which was followed by Louis Agassiz in 1860. Nichol son has used the term in the reverse sense for a heterogeneous assemblage of those medusae not classified by Huxley as Luccrnaridce, nor as yet recognized as derived from hydroid trophosomes. This use of the term adds to the existing confusion, and renders its abandonment necessary. The term Ltiscomedusce was used for the Scyphomedusoe by Haeckel in his Gencrelle Morphologic (exclud ing Ckarybdcea) whilst Cams (Handbuch, 1867) confines the terra &quot; Medusae &quot; to them alone, which is objectionable, since it belongs as justly to the Hydromeduscr,. Forbes s term for them, Steganoph- thalmia, indicates a true characteristic, failing only in the Luccr- naricc, but its complementary term Gymnophtfialmia is inaccurate. Similarly the terms Acraspcda and its complement Craspcdota are inacceptable. Eimer has proposed to use the terms Toponeura and Cycloneura for the two divisions but Charybdcea appears to break down this division as so many others. The old term Acalcphce, which is retained by Gegenbaur in its proper sense for all the C celentera nematophora, is used as the designation of the Scyiiho- mcdusce alone by Claus (Grundziigc der Zool., 1878), which cannot fail to produce confusion. The term Luccrnaridce, proposed so long ago as 1856 by Huxley (Med. Times and Gazette), most truly indi cates the relationships of these organisms which he was the first to recognize, but it seems desirable to restrict this term to the limited order in which Lucernaria is placed, and to employ for the larger group Scyphomedusce a term which is the true complement of the convenient name assigned to the other division of Hydrozoa, viz., ffydromedusce. 1 Order 1. Lucernaria;, Scyphomedusce devoid of tenta- culocysts, with the aboral pole of the body produced into an adhesive disc by which the organism (which possesses the power of swimming by contraction of the circular muscular zone of the hypostome) usually affixes itself. The enteric cavity is divided into four perradial chambers by four delicate interradial 2 septa. The genitalia are developed as four-paired ridges at the sides of the interradial septa en the oral wall of the chambers (fig. 19). No reproduc tion by fission nor &quot; alternation of generations &quot; is known in the group. At the edges of the disc capitate tentacles are developed in eight adradial 2 groups ; between these are modified tentacles in some genera, the marginal anchors or colleto-cystophors. The canal system which has sometimes been described in them is a product of erroneous observation. A very few genera and species of this order are known. They may be justly called the ccenotype of the medusse (James Clark), and their relationship to the free swimming forms may be compared, as was done by L. Agassiz, to the relationship of the stalked Crinoids to such forms as Coma- tula. Three species are not uncommon on the British coasts. By Milne Edwards the animals forming this group were termed Podactinaria and associated with the Anthozoa. By Leuckart they were termed Calycozoa ; it is only of late that the closeness of their relationship to the Scyphomcduscc has been fully recognized, though long since insisted on by Huxley and by James Clark. Haeckel in his new system of the medusas {Sitzungsber. derjcnaischc GcseUscliaft fur Medicin und Naturwiss., July 26, 1878) adopts for them the term ScyphomcduscK in allusion to their permanently maintaining the distinctive features of the scyphistoma larval form of the Acraspeda^ the term which he adopts from Gegenbaur for our Scypkomediisce. Order 2. Discomedusw. These are Scyphomedusce de veloping as sexual medusiform persons by transverse fission from a scyphistoma, or else directly from the egg. They have eight tentaculocysts, four perradial, four interradial, and sometimes accessory ones (adradial). Four or eight genital lobes (ovaria or spermaria or hermaphrodite) are developed from the endoderm forming the oral floor of the central region of the enteric cavity, which is produced into a corresponding number of pouches. The mouth is either a simple opening at the termination of a rudimentary manubrium (sub-order Cubostomae), or it is provided with four or eight arm-like processes (sub-orders Semostomce and Rhizostomce). In the sub-order Rhizostomce (fig. 24, a), the 1 Scyphomedusce (ffKiupos, a cup) are medusse which are related by strobilation to Scyphistoma,- a wide-mouthed polyp with four gastral ridges. Jfydromeduscc are medusse related to a Hydra, a narrower polyp, devoid of gastral ridges, by lateral gemmation. 8 For use of these terms see paragraphs on Aurelia below.