12 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 



main groups into which the Medust'B are divided. 

 In the "naked-eyed" group, the tubes are un- 

 branched and run in a straight course to the margin 

 of the umbrella, where they open into a common 

 circular tube which runs all the way round the 

 margin (see Figs. 1 and 22). In the " covered-eyed " 

 group, on the other hand, the tubes are strongly 

 branched (see Fig. 8), although they likewise all 

 eventually terminate in a single circular tube. This 

 circular or marginal tube in both cases communi- 

 cates by minute apertures with the external medium. 



The margin of the umbrella, both in the naked 

 and covered eyed Medusre, supports a series of con- 

 tractile tentacles, w^hich vary greatly in size and 

 number in different species (see Figs. 1 and 8). The 

 margin also supports another series of bodies which 

 will presently be found to be of much importance 

 for us. These are the so-called " marginal bodies," 

 w^hich vary in number, size, and structure in 

 different species. In all the covered- e^^ed species 

 these marginal bodies occur in the form of little 

 bags of crystals (therefore they are called " litho- 

 cysts "), which are protected by curiously formed 

 "hoods " or " covers " of gelatinous tissue ; and it is 

 on this account that the group is called " covered- 

 eyed," in contradistinction to the "naked-eyed," 

 where these little hoods or coverings are invariably 

 absent (compare Fig. 1 with Fig. 22), and the 

 crystals frequently so. In nearly all cases these 

 marginal bodies contain more or less brightly 

 coloured pigments. 



The question whether any nervous tissue is 



