CCELENTERATA I HYDROZOA. 



117 



series 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" The annula- 

 tions 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 concave 



Fig. 33. Development of Aurelia, one of the Luceniarida. a. Ciliated free-swim- 

 ming embryo, or "planula;" b Hydra-tuba; c Hydra-tuba in which fi>sion has 

 considerably advanced, and the " Strobila " stage has been reached ; d Hydra-tuba 

 in which the fission has proceeded still further, and a large number of the segments 

 have been already detached to lead an independent existence. 



surfaces upwards. ' This stage was described by Sars under 

 the name of "Strobila" (c). 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 (c). 

 " 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 Escholtz 

 as true Medusidce. under the name of Ephyrcz" (d}> 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 fertilised ova, when 

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



