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MANUAL OF ZOOLOGY. 



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 longitu- 

 dinal 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 Coelen- 

 terata.) The Hydra-tuba thus constitutes the fixed ' Lucer- 

 naroid,' or the * trophosome ' of one of the Rhizostomidce. 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 pro- 

 duced by the following singular process (fig. 22). The Hydra- 



Fig. 22. Development of Lucernarida (Chrysaora). a. Ciliated ovum or ' planula.' 

 &. Hydra-tuba, c. Hydra-tuba undergoing fission or ' Scyphistorua.' 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. /. Free-swimming medusoid or 'Ephyra,' pro- 

 duced by fission from the hydra-tuba. 



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 ' Scypliistoma ' (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 

 concave surfaces upwards. This stage was described by Sars 

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



