140 MANUAL OF ZOOLOGY. 



appearance, until a circlet of numerous filiform appendages, 

 containing thread - cells, surrounds the distal margin of the 

 ' Hydra - tuba ' (b\ as the young organism at this stage of 

 its career has been termed by Sir J. G. Dalyell. The mouth, 

 in the meantime, from being a mere quadrilateral orifice, 

 grows and lengthens itself so as to constitute a true polypite, 

 occupying the axis of the inverted umbrella or disc, which 

 supports the marginal tentacles. The space between the 

 walls of the polypite and umbrella is divided into longitudinal 

 canals, whose relations to the rest of the organism, and, 

 indeed, the whole structure of Hydra-tuba, closely resemble 

 what may be seen in Lucernaria " (Greene, ' Manual of 

 Ccelenterata '). The Hydra -tuba thus constitutes the fixed 

 " Lucernaroid," or the " trophosome " of one of the Rhizosto- 

 midcz. 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. 58). The 

 Hydra-tuba becomes elongated, and becomes marked by a 

 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 

 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 Eschscholtz 

 as true Mcdusida under the name of Ephyrce (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 contraction 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 



