HYDROZOA 



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. 



development have recently formed the subject of investiga- 

 tion by Glaus, Eimer, and others. As the current accounts 



FIG. 24. Seyphomtduscr. a, Rfiiiottoma pulmo; b, Chrfsaora hfotceua 



In the Semostomce and Rhizostomce (not in the Cubostomte) 

 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 



FJG. 25 Fmir stages in the development of Ckiytaora. A, Diblasrnla stage; 

 B, stage after closure of blastopore ; C. fixed larva with commencing stomodajum 

 ororal ingrowth ; D, filed larva with mouth, short tentacles. Ac. ; tp. ectoderm ; 

 Ay. endodenn ; it. stomodxum ; m. mouth ; 6.', blastopore. (From Balfour. after 

 Clans.) 



has been erroneously supposed, with the genital organs, the 

 products of which normally are evacuated by the mouth. 

 In the Tetragamelian Rhizostoirue 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 Semo- 

 ftomue, the somewhat less common Rhizostoma of the 

 Monogamelian Rhizostomae, whilst Xausithoe 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 

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

 finitions which hare been introduced with modifications in relation 

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

 Claus (GrundzUge) for the Disconieduscc. It is quite clear from the 

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

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

 altogether from use in systematic treatises. 



The structure of the commoo Aurelia aurita and its 



FIG. 26. Later development of Chrvsaora and Aurelia (after Clans). A, Scyphi- 

 stoma of CArytaora. with four perradial tentacles and horny basal perisarc. 

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

 of four interradial 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 intemdially 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 Aurelia 

 scyphistoma to form the pile of ephyrse or young medusae (see fig 27). The 

 single epbyra 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 tlie disc 

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

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

 increased to three in each of the four sets. A, margin of the month : Ad. 

 adradial radius: F, gastral filament; In. interradial radius; JG. adradrial 

 gastral canal ; JR=R*. adradial lobe of the disc ; K, lappet of a perradial arm ; 

 JC stomach wall; Hit, muscle of the gastral ridge: Xtr. gastral ridge; 

 J/i, mesoderm; O, tentaculocyst ; P, perradial radius; K 1 , interradial radius; 

 R*. adradial radius ; SG, commencement of lateral vessel. 



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

 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 comple- 

 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 thb stage used 

 to be termed in all Ccelentera), 

 after swimming around for a 

 time, fixes itself, probably by FlG 

 the blastopo^al pole. The true 

 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 pfrradial. Meanwhile 

 the aboral pole narrows and forms a distinct stalk, 

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



Above to left, young scyphistoma 

 with four peira'dial tentacles. Be- 

 low to left, scyphistoma with six- 

 teen tentacles and first constriction. 

 To the light, stiobila 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 ephyrae. 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 Gegenbanr ) 



