630-600 million years ago


As worms became more complex, the muscular system became more important in locomotion, contracted in waves known as peristalsis, and added additional proteins which are shared with higher animals.

The cutaneous musculature of primitive flatworms still possessesed radial symmetry (Beklemishev, vol. 1, p. 108) and even included musculo-epithelial cells similar to those found in cnidarians (Barrington, p. 71). The most primitive and smallest flatworms depended on cilia as their primary means of locomotion; the role of muscle in locomotion increases in larger flatworms which utilized waves of muscle contraction. In nemertine worms, waves of muscle contraction are referred to as peristalsis, as are the waves of muscle contraction used in a number of internal tracts in vertebrates (Barrington, p. 73). The development of a coelom in higher worms had a great impact on the muscular system in that it increased the flexibility and speed of possible movements. Higher bilateran worms (prior to the evolution of nematodes) developed proteins found in the muscles of higher animals such as troponin, tropomyosin, and titin (Hoar, 1983, p. 325; Kenny, 1999; Champagne, 2000).