CHAPTER II. 
FUNDAMENTAL EXPERIMENTS. 
THE naked-eyed Medusz are very much smaller in 
size than the covered-eyed, and as we shall find 
that the distribution of their nervous elements is 
somewhat different, it will be convenient to use 
different names for the large umbrella-shaped part 
of a covered-eyed Medusa, and the much smaller 
though corresponding part of a naked-eyed Medusa. 
The former, therefore, I shall call the umbrella, and 
the latter the swimming-bell, or nectocalyx. In 
each case alike this portion of the animal performs 
the office of locomotion, and it does so in the same 
way. I have already said that this mushroom-like 
organ, which constitutes the main bulk of the 
animal, is itself mainly constituted of thick trans- 
parent and non-contractile jelly, but that the whole 
of its concave surface is lined with a thin sheet of 
muscular tissue. Such being the structure of the 
organ, the mechanism whereby it effects locomotion 
is very simple, consisting merely of an alternate 
contraction and relaxation of the entire muscular 
sheet which lines the cavity of the bell. At each 
contraction of this muscular sheet the gelatinous 
