SUB-CLASSES LUCEENAEinA, GEAPTOLITID^, ETC. 79 



series of marginal tentacles, which are often of extraordinary length. 

 Besides the tentacles, the margin of the umbrella is provided with 

 a number of marginal bodies, each of which consists of a little col- 

 lection of pigment, or " eye-speck," and a little sac filled with fluid 

 and containing mineral particles. Each of these marginal bodies 

 is covered and concealed from view by a kind of hood derived from 

 the ectoderm. Hence the name of " hidden-eyed " Medusae, applied 



Fig. 43.— Generative zooid of one of the Lucernarida {Chrysaora Jij/cscella). 

 (After Gosse.) 



to these forms, in contradistinction to the " naked-eyed " J/edusce, 

 in which the eye-specks are exposed to view. There is also a well- 

 developed nerve-ring running round the mouth of the umbrella. The 

 reproductive organs are usually of some bright colour, and " form a 

 conspicuous cross shining through the thickness of the disc.'' 



From the above description it will be evident tbat there is a considerable 

 re.9eniblance between the so-called " MAden-eyed" Mednsce, or the reproduc- 

 tive zooids of many of the Lucernarida, and the medusiform gonophores of so 

 many of the Hydrozoa, as well as the true Medusidcs or naked-eyed Medusce. 

 The differences, however, between them are these ; The swimming-disc of the 

 naked-eyed Medusce and of any medusiform gonophore is furnished at its mouth 

 with an internal shelf or veil ; the radiating gastro-vascular canals are very 

 rarely more than four in number, and should they subdivide (as in rare cases 



