660-630 million years ago

cnidarian with muscles

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).