HYDEOZOA 



67 



edges of the oral opening fuse together at an early age 

 and leave several sucker-like secondary mouths, which were 

 formerly mistaken for independent persons. The central 

 enteric chamber is continued through the disc by a com- 

 plicated often reticulate system of radiating canals, which 

 excavate the endoderm lamella. 



Fig. 24. — iScyphomedtisa. a, Rhizostoma pulmo ; h, Chrysaora hyoiceua 



In the Semostomce and Rhizostomce (not in the Gubosfomce) 

 four remarkable (respiratory) sub-genital pits (fig. 28) are 

 hollowed out in the gelatinous substance of the sub-umbrella 

 (oral face of the umbrella). These do not communicate, as 



development have recently formed the subject of investiga- 

 tion by Claus, Eimer, And others. As the current accounts 



Fio. 25.— Four stages in the development of Chrysaora. A, Dihlastula stage ; 

 B, stage after closure ofblastopore; C, fixed larva with commencing stomodseuni 

 or oral ingrowth j D, fixed larva with mouth, short tentacles, 4c. ; ep, ectoderm ; 

 hy, endoderm ; st, stomodseum ; m, mouth : bl. blastopore, f From Balfour, after 

 Claus.) 



has been erroneously supposed, with the genital organs, the 

 products of which normally are evacuated by the mouth. 

 In the Tetragamelian Jihizostomoe these pits remain distinct 

 from one another as in Semostomce, but in the Monogamelian 

 RhizostomcB they unite to form one continuous sub-genital 

 cavity placed between the wall of the enteric cavity and 

 the polystomous oral disc. The common English forms, 

 Aurelia, Chrysaora, and Cyarma, are types of the Semo- 

 stomce, the somewhat less common RMzostoma of the 

 Monogamelian Rhazostomoe, whilst Nausithoe and Disco- 

 medusa represent the simple Cubostomce. 



The writer has adopted the term used by Haeckel for this order, 

 and is indebted to his preliminary notices of a large work on the 

 Medusce, now in the press, for outlines of the classification and de- 

 finitions which have been introduced with modifications in relation 

 to these and the other Medusce. The term Discophora is used by 

 Claus (Grundaiige) for the Discomedv^oe. It is quite clear from the 

 varied and inconsistent use by difierent authors of that term, and 

 also of the terms Acalepkoe and Medusce, that they must be ejected 

 altogether from use in systematic treatises. 



The structure of the common Aurelia aurUa and its 



Fig. 26. — Later development of Chrysaora and Aurelia (after Claus). A, Scyphi- 

 stoma of Chrysaora, with four perradial tentacles and homy'hasal perisarc. 

 B, Oral surface of later stage of scyphistoma of Aurelia, with commencement 

 of four inten'adial tentacles. The quadrangular mouth is seen in the centre ; 

 the outline of the stomach wall, seen by transparency around it, is nipped in 

 four places interradially to form the four gastric ridges. C, Oral surface of 

 a sixteen-tentacled scyphistoma of Aurelia. The four gastric interradial 

 ridges are seen through the mouth. D, First constriction of the AureHa 

 scyphistoma to form the pile of ephyne or young medusffi (see fig. 27). The 

 single ephyra carries the sixteen scyphistoma tentacles, which will atrophy 

 and disappear. The four longitudinal gastric ridges are seen by transparency. 

 E, Young ephyra just liberated, showing the eight bifurcate arms of tlie disc 

 and the interradial single gastral filaments. F, Ephyra developing into a 

 medusa by the growth of the adradial regions. The gastral filaments have 

 increased to three in each of the four sets, .il, margin of the mouth; Ad, 

 adradial radius; F, gastral filament; In, interradial radius; JO, adradilal 

 gastral canal ; JJR=R^, adradial lobe of the disc ; K, lappet of a pen-adial arm ; 

 M, stomach wall; Mst, muscle of the gastral ridge; Mw, gastral ridge ,- 

 Ms, mesoderm ; 0, tentaculocyst ; P, perradial radius ; R^, interradial radius ; 

 R^, adradial radius ; SO, commencement of lateral vessel. 



in text-books are very inadequate, a short sketch of the 



morphology of that form is appended here. 

 From the egg, according 



to the researches of Claus 



(whose figures, here repro- nJLJM 



duced, refer more especially V. J 



to the closely allied genus \ / 



Chrysaora, up to the compl e- \L 

 tion of the scyphistoma), a 

 single-cell-layered blastula de- 

 velops which forms a diblastula 

 by invagination (fig. 25, A, B, 

 G). The orifice of invagination 

 closes up, and the ciliated 

 " planula " (as this stage used 

 to be termed in all Coslentera), 

 after swimming around for a 



time, fixes itself, probably by p^^. 27.-Development of AureHa. 

 the blastoporal pole. The true Above to left, young scyphistoma 



mouth then forms by inruption 

 at the opposite pole. Two ten- 

 tacles now grow out near the 

 mouth opposite to one another 

 (fig. 25, D), and are followed 

 by two more (fig. 26), these 

 indicating the four primary 

 radii of the body which pass 

 through the angles of the four- 

 sided mouth, and are termed perradial. Meanwhile 

 the aboral pole narrows and forms a distinct stalk, 

 which in Chrysaora secretes a horny perisarc (fig. 25, 



with four perradial tentacles. Be- 

 low to left, scyphistoma with six- 

 teen tentacles and first constiiction. 

 To the right, strobila condition of 

 the scyphistoma, consisting of thir- 

 teen metameric segments ; the up- 

 permost still possesses the sixteen 

 tentacles of the scyphistoma; the 

 remainder have no tentacles, but 

 are ephyrge, each with eight bifid 

 arms (processes of the disc). Each 

 segment when detached becomes 

 an ephyra, such as that drawn in 

 fig. 26, E, F. (From Gegenbaur.) 



