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Invertebrate
muscle systems. Cnidarians
The
phylum Cnidaria includes the hydras, jellyfishes, and sea
anemones. Cnidarians have two main body forms, the cylindrical
tentacled polyp exemplified by the hydra and the sea anemone,
and the bell-shaped (or inverted saucer-shaped) medusa.
Hydras are some of the simplest multicellular animals to
have muscle. They are hollow, cylindrical, freshwater creatures
about 10 millimetres long. One end attaches to a plant or
some other support, and the other end is free and has a
mouth surrounded by tentacles. The body wall consists of
two layers of cells with a middle gelatinous layer (mesoglea).
In hydras and other two-layered animals one kind of cell
serves as both muscle and epithelial cells. The compact
body of each cell is packed closely with the adjacent cells
to form an epithelium, and the base of each cell, where
it meets the mesoglea, is drawn out into a long muscle fibre.
In the hydra the musculoepithelial cells that cover the
outer surface of the body have longitudinal muscle fibres;
those that line the gut cavity (the gastrodermis) have circular
muscle fibres. Sea anemones have all of the muscle fibres
in the gastrodermis, though some of the fibres are longitudinaland
some are circular. When the mouth of the sea anemone is
closed, the water in the gut cavity acts as a hydrostatic
skeleton, permitting the animal to grow longer and thinner
or shorter and fatter, or to bend in any direction. These
changes result from the interaction of the longitudinal
and circular muscles through movements that are not as simple
as those of the schematic worm of Figure 5. The hydra can
reduce its volume by using its muscles to squeeze water
out of the gut cavity through the open mouth. It can reinflate
using cilia to circulate water into the gut cavity. Its
movements are also influenced by the viscoelastic properties
of the mesogleal jelly.
The
largest and most familiar medusae are the jellyfishes of
the class Scyphozoa, some of which grow to a diameter of
two metres. Though large, the scyphozoan jellyfishes have
only a single layer of cells on the outer surface of the
body and a single layer lining the gut cavity; most of the
volume of the animal is occupied by the gelatinous mesoglea.
The epidermis of the undersurface of the bell includes themusculoepithelial
cells responsible for the animal's weak swimming movements.
The muscle fibres contract, reducing the diameter of the
bell and forcing out a stream of water. The bell thenreturns
to its original shape by elastic recoil of the mesoglea.
These movements are performed in a regular rhythm with a
period of a few seconds, propelling the animal through the
water. Medusae are among the simplest animals that use muscles
to make rhythmic movements. In at least some medusae the
circular muscles, which do most of the work of swimming,
are striated. In contrast, most of the other muscles of
cnidarians are smooth.
