HHI/OSTIIM.B. I'l.l] 



Order RHIZOST01VLE Cuvier, 1799. 



Rhizostom*, CivltR, 1799, Journal de Phys., tome 49, p. j\(>. HAM KM, iSXo, Syr. ,|.-r Mr In . -n, p. 560.- HAMANX, iSSl, 



Jena. Zcit. fur Naturwissen., Bd. 15, p. 24;. CIAI ^, iSS^, t'ntersuch. uber Organisation und Enticl.. ,1, i \I. 



Leipzig, p. 5o; 1886, Arbeit. Zool. Inst. Wien, Bd. 7, p. no. VON LCNDF.NFF.LD, 1888, Zett. fur wissen. /....I., li.t. 4-. 



p. 208. 



Rhixostomea, A<.A-<SI/ , I... iS'>2, p. 208, c'onr. N'.it. Hi*t. I", s.. vol. 4, p. i4< } . 

 Rhizosiomata, VANHOFFLN, 1888, Bibliotheca Zoologica, Bd. i. Heft. 3, p. 39. MAAS, 190-,, Scyphomedusen der SiboRa Eipcd., 



M<iiiog. n, p. 88; 1906, Revue Suis,r ,!, /,<,!., tome 14, p. 100; 1907, Ergeb.und Kortsclirittc der Zoologic, BJ. I, p. 201. 

 Rhizostomidx, ESCHSCHOLTZ, 1829, Syst. der Acali-ptn-n, p. 42. 



CHARACTERS OF THE ORDER. 



Scyphomedusae without marginal tentacles*, ami \\iili numerous mouths which are borne 

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

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

 tentacles. 



The rhopalia and marginal lappets of the Rhi/ostomae are similar to those of Semajos- 

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

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

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

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

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

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

 in the tropical Atlantic and the Mediterranean. The Rhizostonuc develop, in so far as \\i 

 know, through strobilization from scyphostomae. Phylogenetically they are derived from the 

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

 Loboiit-mathe marginal lappets have become greatly elongated and may in some respects function 

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

 Semaeostomeae, but the 4 rays ot the cross soon fork at tlu-ii outer ends and then grow i>m\\.inl 

 in the form of 8 adradial, fleshy, mouth-bearing appendages, which branch in a charaeteiistic 

 manner and constitute the so-called "mouth-arms." The 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-amis. The sem;costomous genus 

 Aufosa with its 4 bifurcated mouth-arms, which in other respects resemble those ot '.inr, l/m, 

 appears to be a connecting link between the Semasostomeas and the Rhizostomx. 



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

 as in Cyanea. In the young medusa; of all Rhizostonue and in the mature medusae of some 



naaaa* genera such as (.'<;< M<if>i-ii there are4sepa- 

 \ rate genital sacs which project into the 



Is -J central stomach-cavity ot the medusa. In 

 many Rhizostomas such as in Cotylorhiza, 

 .\f,i\tifli<i.<, 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 of the 



Fio. 4 o,. Digrams illu.tr.um, ,! i ,,, the 4 primitive me d u sa and not connected with the gas- 

 genital cavities (A) to Form a angle cruciform cavity (c). 



trovascular cavity, but opening to the 



surrounding ocean through the 4 interradial genital ostia. The formation ot this cmcifnim, 

 genital cavity, or subgenital porticus of I laeckel, is illustrated in the 3 diagrams of figure 401. 

 In A, we see 4 separate, interradial, genital sacs ((/') projecting into the stomach-cavity (.S'l, 

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

 species of Cfphea (Netrostoma); and in C we hnd the walls broken down in the fused regions 

 forming a cruciform genital space lying beneath the stomach as in Cotylorhiza, Crambessa, 

 Mtiitigiiis, etc. 



The umbrella of the Rhizostonitc resembles that of their more simply organized ancestors 

 the Semsostomeae. It is usually dome-shaped and covered with nematncvst -waits. The In II- 

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



