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MANUAL OF ZOOLOGY. 



midce. In height it is less than half an inch, but it possesses 

 the power of forming, by gemmation, large colonies, which may 

 remain in this condition for years, the organism itself being 

 incapable of producing the essential elements of generation. 

 Under certain circumstances, however, reproductive zooids are 

 produced by the following singular process (fig. 23). The 



Fig. 23. Development of Lucernarida (Chrysaora). a Ciliated ovum or " planula ; " 

 b Hydra-tuba; c Hydra-tuba undergoing fission, or " Scyphistoma ;" d The fission 

 still further advanced, constituting the " Strobila ; " e A form still further advanced, 

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

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



Hydra-tuba becomes elongated, and becomes marked by a se- 

 ries of grooves or circular indentations, extending transversely 

 across the body from a little below the tentacles to a little above 

 the fixed extremity. At this stage the organism was described 

 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- 

 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 Hydra-tuba 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 Medusida under the name of Ephyra" (f). 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- 



