12 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
main groups into which the Medusz 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 Medusz, supports a series of con- 
tractile tentacles, which 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,” 
which vary in number, size, and structure in 
different species. In all the covered-eyed 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 
