Complex sentence: Each module included a well-developed band of dorso-ventral muscles running from the top to the single, broad, muscular "foot", and smaller transverse ventral muscles from side to side on the underside of the body.
Split into simpler sentences: Each module included a well-developed band of dorso-ventral muscles running from top to bottom. It had a single, broad, muscular "foot" and smaller transverse ventral muscles from side to side on the underside of the body.