divisiojSts of the hydeozoa. 71 



the whole system constituting the so-called " gastro- vascular " or 

 " iiectoealycine " canals. The margin of the bell is narroweiJ by a 

 kind of shelf, which runs round the whole circumference, leaving a 

 central aperture, and which is known as the " veil." From the mar- 

 gin of the disc hang more or less numerous tentacles, which in these 

 forms are mostly solid, instead of being hollow. Also round the 

 circumference of the swimming-bell are disposed certain " marginal 

 bodies," which are doubtless organs of sense. Some of these marginal 

 bodies consist of little rounded sacs or " vesicles," filled with a trans- 

 parent flviid, and containing mineral particles, apparently of carbon- 

 ate of lime. These are pjobablj' rudimentary organs of hearing. 

 Others of the marginal bodies are in the form of little masses of 

 colouring-matter or pigment, often of a strikingly bright colour, en- 

 closed in distinct cavities. These are known as the "pigment-spots" 

 or " eye-specks," and they are of the nature of rudimentary organs 

 of vision. They are placed in a conspicuous and unprotected position 

 on the margin of the disc, and hence these organisms were termed 

 " naked-eyed " Medusae by Edward Forbes. Bound the margin of 

 the swimming-disc there is also jjlaced a distinct nerve-ring. The 

 reproductive organs (fig. 38) are usually developed in the coiirse of 

 the radiating gastro-vascular canals, but are sometimes situated in 

 the walls of the central polj'jjite. 



The naked-eyed Medusce, including under this name both the true 

 JR'dusa; and the free medusoid buds of other Hydrozoa, are all exceed- 

 ingly elegant and attractive, when examined in a living condition, 

 resembling little bells of the most transpai-ent glass, adorned here and 

 there with the most brilliant colours. They occur, in their projier 

 localities and at proper seasons, in enoi-nious nundiei'S. They are 

 mostly phosphorescent, or capable of giving out light at night, and 

 they appear to be one of the principal soiu'ces of the luminosity of 

 the sea. It does not seem, however, that they phosjjhoresce unless 

 disturbed or irritated in some way. 



Most Medusfje and medusoid gonojihores are of comparatively 

 small size, but some types of the latter may grow to a diameter of 

 eight or ten inches or more (<?.</., JEquorvd). A single Medusoid 

 {Limnocodium) has been found inhabiting fresh water ; but this is 

 known to be the generative bud of a fixed Hydroid. 



