I02 



MANUAL OK ZOOLOGY. 



simple, or it may be produced into lobes, which are most 

 frequently four in number. The essential elements of genera- 

 tion are produced in simple expansions either of the wall ol 

 the manubrium cr of the radiatmg nectocalycine canals. 



Fig. 22. — Group of naked-eyed Medusis. A Sarsia gemmifera, with meduso'.ds 

 arising: froni the sides of the central polypite (after Greene) ; B Madceria formosa 

 (after Forjaes); C /'(7/j'jr^«:Vz ^/(/^n (after Gosse). 



From the above description it will be evident that the 

 Medusa is in all essential respects identical in structure with 

 the free-swimming generative biid or gonophore of many of 

 the fixed and oceanic Hydrozoa. Indeed, a great many forms 

 which were previously included in the Medusidcs have now 

 been proved to be really of this nature, and it may fairly be 

 doubted if this will not ultimately be found to apply to all. 

 As to the value, however, of the order Medusidce, the present 

 state of our knowledge is well expressed by the following con- 

 clusions which have been drawn up by Professor Greene : — 



" I. That several of the organisms formerly described as 

 MedusidcB are the free gatiophores of other orders of Hydrozoa. 



" 2. That the homology of these free gonophores with those 

 simple expansions of the body-wall which in Hydra and some 

 other genera are known to be reproductive organs by their 

 contents alone, is proved alike by the existence of numerous 

 transitional forms and by an appeal to the phenomena of their 

 development. 



