66 



HYDROZOA 



diblastula in all cases, as yet observed, is formed by in- 

 vagination, the blastopore closing up (Balfour). 



Fig. 23. 



Fig. 20. Charyidcea marsupialis (natural size, after Claus). The four annulated 



tentacles are seen deuending from the four lappets placed at the four comers of 

 the quadrangular umhrella. These are interradial. Two of the four pen'adial 

 enteric pouches of the umbrella, representing radiating canals, are seen of a pale 

 tint. Fg, gastral filaments (interradial); B, the modified peiTadial tentacles 

 forming tentaculocysts ; O, comer ridge facing the obseiTer and dividing 

 adjacent pouches of the umbrella; GF, position of one of the genital bands. 



Fig 21.— View of the margin of the umbrella of Chai-ybdaa marsupialis (natural 

 size, after Claus). At the four comers 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 CharybdcBa 

 marsupialis (modified from Claus). Ma, manubrium; SB, side ridge (perradial); 

 CB, comer ridges, separated by CO, the interradial comer groove ; Gfe, the 

 genital lamellae in section, projecting from the interradial angles on each side 

 into UF, the enteric pouches of the umbrella; StT, the sub- umbrella space. 



Fig. 23. — ^Vertical sections of Charybdtea marsupialis, to the left in the plane of 

 an interradius, to the right in the plane of a pen-adius. ifa, manubrium; 

 FAX, axial enteron; Gh, gastral filaments (phacellis); CG, corner groove; 

 SB, side ridge ; FnL, endoderm lamella (line of concrescence of the walls of 

 the enteric cavity of the umbrella, whereby its single chamber is brollen up into 

 four pouches) ; Ge, line of attachment of a genital band; FU, 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 TOa, the tentacular 

 canal; 7e, velum; i?'r, freuum of the velum ; Tfc, tentaculocyst. 



The binaty division of the Sydrozoa was established by Esoh- 

 scholtz [System der Acalephen, 1829) whose DiscophorcB pTumsro- 

 carpce correspond to the SeypJiomediisce, whilst his XHscqphorce 

 eryptocarpce represent the Sydromedusce. The terms point to dis- 

 tinctions which are not valid. In 1853 Kolliker used the term Dis- 



cophora for the Scyphomedusce alone, an illegitimate limitation of 

 the term which was followed by Louis Agassiz in 1860. Nichol- 

 son has used the term iu the reverse sense for a heterogeneous 

 assemblage of those medusse not classified by Huxley as Lucemarid(e, 

 nor as yet recognized as derived from hydroid trophosomes. This 

 use of the term adds to the existing confusion, and renders its 

 abandonment necessaiy. The teim Discomedusce was used for the 

 Scyphomediism by Haeckel in his Oenerelle Mmphologie (exclud- 

 ing (7Aarj/M(Ka)— whilst Cams [Handbueh, 1867) confines the term 

 " Medusae " to them alone, which is objectionable, since it belongs 

 as justly to the Hydromedmsm. Forbes's term for them, Stegwiwph- 

 thalmia, indicates a true characteristic, failing only in the Liicer- 

 nariae, but its complementary term GymnopMhahnia is inaccurate. 

 Similarly the terms Aaraspeda and its complement Craspcdota are 

 inacceptable. Eimer has proposed to use the terms Toponeura and 

 Gycloneura for the two divisions — but Charybdcea appears to break 

 down this division as so many others. The old term Acalephoe, 

 which is retained by Gegenbaur in its proper sense for all the 

 CodenUra nematophora, is used as the designation of the Scypho- 

 medusce alone by Claus (Grundziige der Zool., 1878), which cannot 

 fail to produce confusion. The term Lucemaridce, 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 — ScyphomeduscB — a term which is the true complement of 

 the convenient name assigned to the other division of Hydrozoa, 

 viz., ffydromedusce.'^ 



Order 1. Lucernarioe, — Scyphomecktsm 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^ septa. The genitalia are developed 

 as four-paired ridges at the sides of the interradial septa 

 on the oral wall of the chambers (fig. 19). No reproduc- 

 tion by fission nor " alternation of generations " is known 

 in the group. At the edges of the disc capitate tentacles 

 are developed in eight adradial ^ groups ; between these are 

 modified tentacles in some genera, — the marginal anchors 

 or coUeto-cystophors. Thecanal 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 coenotype of the medusae 

 (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 iiie Anthozoa. By Leuckart they 

 were termed Calycozoa ; it is only of late that the closeness of their 

 relationship to the ScyphoTncdusce has been fully recognized, though 

 long since insisted on by Huxley and by James Clark. Haeckel in 

 his new system of the medusffi (Sitzungsber. derjenaische Gesellschaft 

 fur Medicin und Naturwiss., July 26, 1878) adopts for them the 

 term Scyphomediisai in allusion to their permanently maintaining the 

 distinctive features of the scyphistoma larval form of the Acraspedce, 

 the term which he adopts from Gegenbaur for our Sq/pJwmedusce. 



Order 2. Discomedusm. — These are Scyphomedusce de- 

 veloping as sexual medusif orm 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 Cuhostomce), or it is provided with 

 four or eight arm-like processes (sub-orders Semostomm and 

 Rhizostomoe). In the sub-order Rhizostomoe (fig. 24, a), the 



' Scyphomedusce {aKifos, a cup) are medusse which are related by 

 strobilation to Scyphistoma, — a wide-mouthed polyp with four gastral 

 ridges. Hyd/romedusce are medusae related to a Hydra, — a narrower 

 polyp, devoid of gastral-ridges, — by lateral gemmation. 



' For use of these terms see paragraphs on AweUa below. 



