Ancestral cnidarians evolved true muscle cells. A
number of the genes that cnidarians use to develop their muscle are also
used by higher animals. While cnidarians seem to lack a tissue layer (mesoderm)
which is found in bilateran animals, the genetic foundations of this tissue
are present in cnidarians.
Cnidarians possess primitive muscle cells in circular, longitudinal,
and sometime oblique layers (Fretter, p.56-8). A significant amount of
cnidarian muscle is striated. Some invertebrate muscle possesses oblique
striations while others possess striations on one half of the cell and
mitochondria on the other (Prosser, 1973, p. 723; Hoar, 1983). These contractile
cells are not organized into prime movers and antagonists, as in vertebrates
(Hickman, p. 137; Fretter, p. 66). Ctenophores, which some analyses identify
as the group of cnidarians which are most closely related to bilateran
animals, possess true muscle cells (Hickman, p. 181). Skeletal muscle
is mononucleated in cnidarians and worms but multinucleated in vertebrates
and other coelomates such as insects and leeches (Castanon, 2002). The
following slides are of the muscle tissue of a sea anemone
Cnidarians are usually classified as diploblastic (having only two embryonic
tissue layers, ectoderm and endoderm), while all bilateran animals are
classified as triploblastic (because in addition to ectoderm and endoderm,
they possess a third tissue layer, mesoderm, from which muscle is derived).
Although cnidarians are classified as diploblastic, in some cnidarians
the muscle cells of the oral disk are covered by epithelia, not unlike
the location of the mesodermal muscle of flatworms (Barrington, p. 62).
The gene Twist functions in the differentiation of mesoderm in tribloblast
bilaterans but it is also present in diploblast cnidarians. Twist expression
first occurs in the entocodon, the mesoderm-like layer from which muscle
tissue differentiates in cnidarians (Castanon, 2002). The development
of striated and smooth muscle from this third cell layer have led some
to consider cnidarians as tribloblastic (Muller, 2003, Seipel, 2005).
In bilateran animals, some of the genes which are essential for the embryonic
formation of muscle are members of the basic helix-loop-helix gene family
(bHLH). Jellyfish are known to possess at least four bHLH transcription
factors. The sequence, dimerization, and expression of the JellyD1 protein
in striated muscle indicate that it is a homolog of MyoD in bilaterans.
Vertebrate MyoD genes, which can form both homodimers and heterodimers,
can form dimers with JellyD. This indicates that the striated muscle of
jellyfish is homologous to that of bilaterans (Muller, 2003).