788 THE ANIMAL KINGDOM. 



make their appearance opposite to one another, then the remaining two, 

 finally the interradial tentacles and taeniolae simultaneously. The planula 

 of Pelagia developes direct into an Ephyra \ 



Most Acraspeda are pelagic ; a few deep-sea forms are known. The 

 Depastridae and Lucernaridae attach themselves by their peduncle to 

 algae, &c. In some instances the Medusa frequents the surface of mud- 

 banks and coral reefs, and often rests reversed, i. e. upon the convexity of 

 the bell 2 . Cassiopeia polypoides from the coral banks of the Red Sea has 

 an exumbrellar groove which acts as a sucker. The ectoderm cells of the 

 groove secrete a plentiful mucus which cements the coral sand, and there 

 is also an exumbrellar radial musculature. The Tesseroniae are for the 

 most part small : the Cubomedusan Tamoya attains, however, a diameter 

 of eight inches. Many Ephyroniae are large, e. g. Aurelia aurita four 

 inches, and the Rhizostome Pilema pulmo ( = Rhizostoma Cuvierf)two feet: 

 ; but the Semostome family Cyaneidae contains the largest known Medusae. 

 An old example of Cyanea arctica has been measured with a bell seven 

 and-a-half feet across and tentacles a hundred and twenty feet long. Many 

 Acraspeda are brilliantly coloured, and some of them, e. g. Cotylorhiza, are 

 inhabited by the symbiotic alga Zooxanthella (see pp. 242-4). Most of 

 the fossil Medusae mentioned above, p. 746, belong to this sub-class. 



Haeckel classifies the Acraspeda as follows in his System : 



I. Tesseroniae : rhopalia four or none ; stomach surrounded by four wide per- 

 radial gastric pouches separated by longer or shorter septa ; genital organs lodged 

 either in the subumbrellar wall or in the cavity of the gastric pouches. Bell of 

 great depth, usually conical. There are three orders. 



(i) Stauromedusae : no rhopalia; includes Tesseridae with Depastridae, and 

 Lucernaridae. (2) Peromedusae with four interradial rhopalia. (3) Cubomedusae 

 with four perradial rhopalia and lamellate genitalia, e.g. Charybdaea. 



II. Ephyroniae : rhopalia eight, i. e. four per- and four inter-radial, rarely more 

 numerous; stomach surrounded by 8, 16, or 32 gastric pouches or radial canals; 

 genitalia attached to the subumbrellar wall of the central gastric cavity. Bell flat ; 

 for the most part disc-like. One order, Discomedusae, with the above-detailed 

 characters ; three sub-orders as follows : 



(i) Cannes tomae : mouth simple, square; tentacles solid and usually short; 

 the Ephyridae and Linergidae. (2) Semostomae : mouth a cross, the four oral 

 angles prolonged into long arms ; without a circumferential canal, Pelagiidae, e. g. 

 Chrysaora, and Cyaneidae ; with one, Flosculidae and Ulmaridae-, the last-named 



1 It is possible that the planula of Chrysaora may multiply by fission or gemmation : cf. Claus, 

 Untersuchungen, p. 5, and Dk. Wien. Akad. xxxviii. p. 7. Haeckel has described such phenomena 

 together with remarkable forms of Scyphostoma, and even a direct development similar to that of 

 Pelagia in his ' Metagenesis und Hypogenesis von Aurelia aurita' Jena, 1881. Claus in his 

 Untersuchungen criticises (passim) Haeckel's statements unfavourably. Note his corrigendum, 

 p. 90. 



2 The young Cotylorhiza may attach itself in the same fashion, according to Keller. 



