Complex sentence: Their most distinctive feature is the "combs", groups of cilia they use for swimming, and they are the largest animals that swim by means of cilia--adults of various species range from a few millimeters to in size.
Split into simpler sentences: Their most distinctive feature is the "combs"--groups of cilia which they use for swimming--they are the largest animals that swim by means of cilia. Adults of various species range from a few millimeters to in size.