STRUCTURE OF THE MEDUSZ&. 11 
extends to a tolerably close imitation by different 
species of the various forms which are characteristic 
of different species of mushrooms, from the thimble- 
like kinds to the saucer-like kinds. Moreover, 
this accidental resemblance to a mushroom is in- 
creased by the presence of a central organ, occupy- 
ing the position of, and more or less resembling 
in form, the stalk of a mushroom. This organ is 
called the “manubrium,” on account of its looking 
like the “handle” of an umbrella, and the term 
“umbrella” is applied to the other portion of the 
animal. The manubrium, like the umbrella, varies 
much in size and shape in different species, as a 
glance at any figures of these animals will show. 
Both the manubrium and umbrella are almost 
entirely composed of a thick, transparent, and non- 
contractile jelly; but the whole surface of the 
manubrium and the whole concave surface of the 
umbrella are overlayed by a thin layer or sheet 
of contractile tissue. This tissue constitutes the 
earliest appearance in the animal kingdom of true 
muscular fibres, and its thickness, which is pretty 
uniform, is nowhere greater than that of very thin 
paper. 
The manubrium is the mouth and stomach of the 
animal, and at the point where it is attached to or 
suspended from the umbrella its central cavity 
opens into a tube-system, which radiates through 
the lower or concave aspect of the umbrella. This 
tube-system, which serves to convey digested ma- 
terial and may therefore be regarded as intestinal 
in function, presents two different forms in the two 
