Phylum  Platyhelminthes (Flatworms)

Wikipedia Entry

Approximate Number of Species: 20,000

Largest: Diphyllobothrium latum, the human tapeworm, at a length  65 feet (20 meters)!

Smallest: Cyrdodactylus funduli at a length of  0.1 inches (0.258mm)

References:

Brusca RC & Brusca GJ (2003) Invertebrates (Sinauer Associates, Inc., Publishers, Sunderland, MA) 2nd Ed.

Kaestner A (1984) Lehrbuch der Speziellen Zoologie (G. Fischer, Stuggart) 4th Ed.

Yamaguti S (1963) Systema Helminthum, Volume IV, Monogenea and Aspidocotyla (Interscience Publishers, London).