CCELENTERATA : HYDROZOA. 



97 



cretions apparently of carbonate of lime immersed in a 

 transparent fluid. The second class of marginal bodies, vari- 

 ously termed " pigment spots," " eye-specks," or " ocelli," 

 consists of little aggregations of pigment enclosed in distinct 

 cavities. The " vesicles" are probably rudimentary organs of 

 hearing, and possibly the eye-specks are a rudimentary form of 

 visual apparatus. The oral margin of the polypite may be 

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

 quently four in number. The essential elements of generation 

 are produced in simple expansions either of the wall of the 

 manubrium or of the radiating nectocalycine canals. 



Fig. 21. Group of naked-eyed Medusa. A Sarsia gentmifera, with medusoids 

 arising from the sides of the central polypite (after Greene); B Modeeria. formosa 

 (after Forbes) ; C Polyxenia Alderi (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 bud or gonophore of many of the fixed 

 and oceanic Hydrozoa. Indeed, a great many forms which 

 were previously included in the Medusidce 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 Medusida, the present state of our 

 knowledge is well expressed by the following conclusions which 

 have been drawn up by Professor Greene : 



VOL. i. G 



