. A manual of zoology for the use of students : with a general introduction on the principles of zoology . Zoology. 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 advance

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. A manual of zoology for the use of students : with a general introduction on the principles of zoology . Zoology. 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