RHIZOSTOM.E. 



(VM 



Order RHIZOSTOMiE Cuvier, 1799. 



Rhizoiloma:, Cuvier, 1799, Journal dc Phys., tome 49, p. 436.— Haeckel, 1880, Syst. diT Mcdusen, p. 560.— Hamann, 1881, 

 Jena. Zeit. fiir Naturwissen., Bd. 15, p. 243. — Ciaus, 1883, Untersuch. iiber Organisation und Entwick. dcr Meduscn, 

 Leipzig, p. 60; 1886, Arbeit. Zool. Inst. Wiin, Bd. 7, p. 110. — von Lendenfeld, 1888, Zeit. fiir wisscn. Zool., Bd. 47, 

 p. 208. 



Rhnoiionieir, .AfiASSiz , L., 1S62, p. 208, Cont. Nat. Hist. U. S., vol. 4, p. 149. 



Rhizostoinata, VANiitJFFEN, 1888, Bibliotheca Zoologica, Bd. i, Heft. 3, p. 39. — Maas, 1903, .Scvpliomcdusen der Sihof^a Expcd., 

 Monog. II, p. 88; 1906, Revue Suisse de Zool., tome 14, p. 100; 1907, Ergcb. und Fortscliritte der Zoologie, Bil. I, p. 201 . 



RhizostomidcXy Eschscholtz, 1829, Syst. der Acalephen, p. 42. 



CHARACTERS OF THE ORDER. 



Sc\phoniedusx without marginal tentacles*, and with numerous mouths which are home 

 upon 8 adradial, fleshy, branched arm-like appendages which arise from the center of the 

 subumbrella. The lips ot the numerous mouths are bordered by minute, constantly moving 

 tentacles. 



The rhopaha and marginal lappets of the Rhizostomae are similar to those of Semjeos- 

 tomeae. The Rhizostomae are the most highly differentiated Scyphomedusa;, and owing to 

 the generally tough consistency of their gelatinous substance and their large size they have 

 often been found preserved as lossils, especially in the lithographic slates of Solenhofen and 

 Eischstiidt. They are tropical forms and none are known from the polar regions. The 

 genus Rhizostoma is the only one which extends far into temperate regions, and the majority 

 of the wenera are confined to the warm waters of the Indo-Pacific region. A few are found 

 in the tropical Atlantic and the Mediterranean. The Rhizostomae develop, in so far as we 

 know, through strobilization from scyphostoma. Ph}logenetically they are derived from the 

 more simply organized Semaeostomeas, but they have lost their marginal tentacles, though in 

 Lohoiniiia the marginal lappets have become greatly elongated and ma\ in some respects function 

 as tentacles. The ephyra of the Rhizostomae has a simple cruciform, central mouth, as in the 

 Semaeostomeae, but the 4 rays of the cross soon fork at their outer ends and then grow outward 

 in the form of 8 adradial, flesh\-, mouth-bearing appendages, which branch in a characteristic 

 manner and constitute the so-called "mouth-arms." 1 he primitive central mouth may then be 

 obliterated by the coalescence of its lips, but numerous other mouth-openings remain in the 

 gutter-like grooves which extend down the ventral sides of the mouth-arms; and these mouths 

 may also extend over parts of the dorsal sides of the mouth-arms. The semaeostomous genus 

 Aurosa with its 4 bifurcated mouth-arms, which in other respects resemble those of Aurfllui, 

 appears to be a connecting link between the Semaeostomeae and the Rhizostoms. 



The gonads of the Rhizostomae are invaginated as are those of Aiirellia, not protrusive 

 as in C\anea. In the \oung medusae of all Rhizostomae and in the mature medusae of some 



genera such as Cassiopea there are 4 sepa- 

 rate genital sacs which project into the 

 central stomach-cavity of the medusa. In 

 many Rhizostomae such as in Cotylorliiza, 

 Mastigias, etc., the inner ends of the 4 

 primitive, genital bags may coalesce and 

 their walls break down, forming a cruci- 

 form cavity beneath the stomach ot the 

 medusa and not connected with the gas- 

 trovascular cavity, but opening to the 

 surrounding ocean through the 4 interradial genital ostia. The formation of this cruciform, 

 genital cavity, orsubgenital porticus of Haeckel, is illustrated in the 3 diagrams of figure 401. 

 In A, we see 4 separate, interradial, genital sacs (G) projecting into the stomach-cavity (S), 

 as in Rhizostoma and Cassiopea. In B we see these 4 sacs fused at their inner ends as in some 

 species oi Ccphea {Neirostomn); and in C we find the walls broken down in the fused regions 

 forming a cruciform genital space l)ing beneath the stomach as in Cotylorhiza, Cramhessa, 

 Mastigias, etc. 



The umbrella of the Rhizostoma: resembles that of their more simply organized ancestors 

 the Semaeostomes. It is usually dome-shaped and covered with nematocyst-warts. The bell- 



* Excepting in Lobonema, gen. nov., wherein the marginal lobes are converted into long, tapering tentacle-like organs. 





B 



Fig. 401. — Diagrams illustrating the fusion of the 4 primitive 

 genital cavities (a) to form a single cruciform cavity (c). 



