CHAPTER II. 



FUNDAMENTAL EXPERIMENTS. 



The naked-eyed Medusse 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 w^hereby 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 



