Het Leven der Dieren, Deel 3, Hoofdstuk 6: De Wormen

The chapter surveys the diverse animal group traditionally called worms, noting how classification lines have shifted and focusing on the core classes of annelids, roundworms, and flatworms. It outlines external traits such as bilateral symmetry, soft skin often bearing cilia, a skin-muscle sac, and varied body shapes, and explains locomotion by muscular waves, bristles, or suckers. Internal organization ranges from forms without a gut or body cavity to those with a complete alimentary canal and true coelom; nervous systems range from a brain with a ventral nerve cord to simple longitudinal nerves. Circulatory, respiratory and reproductive systems vary widely with lifestyle.