The beating of a
jellyfish umbrella is caused by the contraction and relaxation of the muscles
in the epithelium behind the umbrella. There are fibers of striated muscle on
the back of Aurelia aurita umbrella that
run concentrically. When they contract, the umbrella becomes deflated, and when
they relax, the elasticity of the thick mesoglia above the umbrella restores them
to the original open shape. Jellyfish can swim in the water by repeating this
action. Aurelia aurita at the ephyra larval
stage has radial muscles, which run radially, in addition to the concentric
circular muscles, but the radial
muscles degenerate as they grow. On the other hand, the radial muscles remain
even after maturity in Cyanea capillata.
The beating cycle becomes longer (slower) as it grows, and it changes depending
on the water temperature. With individual differences in the beating cycle, those
jellyfish beating the umbrella vigorously grow well because their tentacles on
the edge of the umbrella flutter well with each beat, collecting and eating a
lot of food.
Each muscle
fiber is contained in a muscle cell. Muscle fibers contain actin filaments and
myosin filaments, and myosin, the main protein component of the myosin
filaments, binds to the actin filaments, pulling them, as it decomposes
ATP. Then, the actin and myosin filaments slide against each other, causing the
muscle fibers to contract. The striated muscle of A. aurita has a striated structure similar to the striated muscle
of the Bilateralia, despite some different characteristics. One of the
differences is that the length of its sarcomere, which is a repeating unit of
striated muscle, is shorter (1.5 micrometers) than those of many Bilateralia (2.5
micrometers). Comparing the case in which muscle fibers of the same length are
composed, there are those with many short sarcomeres, while there are a small
number of long sarcomeres, and the shorter sarcomere muscle fibers have a
higher overall shortening rate even if the actin and myosin filaments slide at
the same rate. In other words, the striated muscle of Aurelia aurita is thought to have a mechanism that allows the
entire muscle to contract quickly, even if the actin and myosin filaments slide
at a low speed. Another characteristic is the Z‐line morphology,
which looks very dark in the Bilateralia when viewed under an electron
microscope. However, it gives the impression of discontinuity and fragility in jellyfish.

On the
other hand, the tentacles and oral arms have smooth muscles. The oral arms are folded in half, and food, excrement, eggs, sperm,
and inedible objects caught by mistake move on the inner side. These take
different routes: sperm are sent from the testes to the tip of the oral arm through the innermost part of the double-folded groove
and released into the sea, while the eggs exit the ditch near the mouth. These
are caused by the movement of the cilia in the inner epithelium. The inner and
outer epithelia are quite different in their cell morphology, and smooth muscle
fibers are located in the outer epithelial cells. This is very different from
the striated muscle, which has separate epithelial cells and muscle cells. Smooth
muscle fibers appear to be reticulated and connected together throughout the oral
arm, but are actually divided into individual epithelial cells.
