34 NATUEAL HISTORY OF 



rescence very graphically in his " Devonshire Coast." 

 He states that going into his study after dark, he took 

 a stick, and felt at random about the water in which 

 his captive hydroids were confined. Presently he 

 touched something soft, and instantly a circle of bright 

 little lamps was lighted up like a coronet of sparkling 

 diamonds, or hke the circular figure of gas jets lighted 

 at a public illumination. He states in another place, 

 that the luminosity of a species of Medusa observed by 

 him, reminded him of the ring of glory in the pictures 

 of the Italian School round the heads of saints. It 

 would appear that the sub -order Athecata does not 

 include any phosphorescent species.* There can be 

 no doubt that much of the apparent phosphorescence 

 is due to the presence of infusoria and other organisms 

 which shelter among the calycles and upon the stems of 

 the hydroida, as on several occasions I have found dis- 

 plays of this quality, which have apparently proceeded 

 from the Zoophytes, to be due to other organisms. 



CLASSIFICATION OF HYDROIDA. 



The order Hydroida contains all the forms which 

 have been assigned by Huxley, Greene, and others to 

 the sub-orders Hydridse, Corynidae, and Sertularidae. 

 The order, as so extended, contains all those Hydrozoa 

 whose hydrosoma or trophosome is either free and 

 consists of a single locomotive polyp, or is fixed and 

 consists of one polyp or of several connected by a 

 coenosarc, with or without hydrothecae, and often 

 developing a polypary or firm outer layer. The re- 

 productive organs appear either as, in the Hydra, 

 simple processes from the body walls, or as gonophores. 

 * Allman, " Gymnoblastic Hydroids," 146. 



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