HYDROZOA 



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. ScyphomedusK. a, Rhizostoma p ulmo- b, Chrysaora hyosceita 



In the Semostoma? and Rhizostomce (not in the Cubostomit) 

 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 



FIG. 25. Four stages in the development of Chrysaora. A, Diblastula stage ; 

 B, stage after closure ol blastopore; C. fixeil larva with commencing stomodieum 

 or oral ingrowth ; D, fixed larva with mouth, short tentacles, Ac. ; 'ep, ectoderm ; 

 hy, endoderm ; st, stomodjeum ; m, mouth ; &/, blastopore. (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 Rhizostomce these pits remain distinct 

 from one another as in Semostomce, but in the Monogamelian 

 Rhizostomce 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 Cyancea, are types of the Seino- 

 stonue, the somewhat less common Rhizostoma of the 

 Monogamelian Rhizostomw, whilst Nausithoe and Disco- 

 medusa represent the simple C'ubostomce. 



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 

 Medusa:, 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 Medusa:. The term Discophora is used by 

 Glaus (Gnuulzilije) for the Discomcduscc. It is quite clear from the 

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

 also of the terms Acalephce and Medusa, that they must be ejected 

 altogether from use in systematic treatises. 



The structure of the common Aurelia aurita and its 



FIG. 21!. Later development, of Chrysaora and Aurelia (after Claus). A, Scyphi- 

 stoma of Cltrysaora, with four perradial tentacles and horny basal perisarc. 

 B, Oral surface of later stage of scyphistoma of Am elia, with commencement 

 of four intervadial 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. I>, First constriction of the Aurelia 

 [ scyphistoma to form the pile of ephyrje or young medusie (see fig 27). The 

 single ephyra carries the sixteen scyphistoma tentacles, which will atrophy 

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

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

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

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

 Increased to three in each of the four sets. ^4, margin of the mouth; Ad, 

 adradial radius; F, gastral filament; //;, interradial radius; JG, adradrial 

 gastral canal ; Jfi=R 3 , adradial lobe of the disc ; K, lappet of a perradial arm ; 

 M, stomach wall; Afst, muscle of the gastral ridge; J/(P. gastral ridge; 

 J/s, mesoderm ; 0, tentaculocyst ; P, perradial radius ; A'-, 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- 



duced, refer more especially \ / 



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, 

 C). The orifice of invagination 

 closes up, and the ciliated 

 " planula " (as this stage used 

 to be termed in all Ceelentera), 

 after swimming around for a 

 time, fixes itself, probably by FlG 27 ._ DcveIopment of Aurelia . 

 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 pel radial tentacles Be- 

 low to left, scyphistoma with six- 

 teen tentacles and first constiiction. 

 To the light, stiobila condition of 

 the scyphistoma, consisting of thir- 

 teen metameric segments ; the up- 

 permost Still pn-^^-l-s Illr Mxtrcn 



tentacles of the scyphi>toma; the 

 remainder have no tentacles, but 

 are ephyrje, each with eight bifid 

 arms (processes of the disc). Each 

 segment when detached becomes 

 an ephyra, such as that drawn in 

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



