CCELENTERATA : HYDROZOA. 



107 



as new by Sars, under the name " Scyphistoma " (c). The 

 annulations or constrictions go on deepening, and become 

 lobed at their margin, till the Scyphistoma assumes the aspect 

 of a pile of saucers, arranged one upon another with their con- 



Fig. 24. — Development of Lucernarida (Chrysaora). a Ciliated embryo or " plaiiula ; " 

 o Hydra-tuba ; c Hydra-tuba undergoing fission, or " Scyphistoma ; " rfThe fission 

 still further advanced, constituting the * Strobila '," e A forni still further advanced, 

 in which a fresh circlet of tentacles has been developed near the base ; / Free- 

 swimming medusoid or " Ephyra," produced by fission from the hydra-tuba. 



cave surfaces upwards. This stage was described by Sars 

 under the name of " Strobila " (d). The tentacular fringe which 

 originally surrounded the margin of the Hydrortuba now dis- 

 appears, and a new circlet is developed below the annulations, 

 at a point a little above the fixed extremity of the Strobila (e). 

 " The disc-like segments above the tentacles gradually fall off, 

 and, swimming freely by the contractions of the lobed margin 

 which each presents, they have been described by Eschscholtz 

 as true MedusidcR under the name of Ephyra " (/). Each 

 Ephyra, however, soon shows its true nature by becoming 

 developed into a free-swimming reproductive body, usually of 

 large size, with umbrella, hooded lithocysts and tentacles, 

 constituting, in fact, a Steganophthalmate Medusa. The re- 

 productive zooid now swims freely by the contractions of its 

 umbrella, and it eats voraciously and increases largely in size. 

 The essential elements of generation are then developed in 

 special cavities in the umbrella; and the fertiHsed ova, when 

 liberated, appear as free-swimming, ciliated " planulse," which 

 fix themselves, become Hydra-tubm, and commence again the 

 cycle of phenomena which we have above described. 



As regards the size of these reproductive zooids as compared 

 with the organism by which they are given off, it may be men- 

 tioned that the umbrella of Cyanea ardica has been found in 



