Platyhelminthes

Overview

Platyhelminthes (Greek: (platys) "flat", (helmins) "worm") is a Phylum within Kingdom Animalia. It is composed of over 34,000 species; notable members of this phylum are flatworms, tapeworms, and planarians.

Characteristics of Platyhelminths*

• Dorsoventrally Flattened
  
• Bilateral Symmetry
  
  • Can be divided into two equal halves.
• Acoelomate
  • No internal body cavity (coelom) other than a simple gut.
• Triploblastic
  • Composed of three tissues
    • Endoderm
    • Mesoderm
    • Ectoderm
• Organ level organization
• Most Classes are Parasitic
• Protonephridia
  • Proto- "first", nephridia (nephros) "kidney"
  • System of tubules that regulates oxygen and metabolic wastes
• Most Classes are Monoecious
  
  • Mono- "one", -ecious (oikos) "house"
    
  • Individuals have both sperm and eggs
• Cephalization and an elemental ganglion

Classes Within Platyhelminthes

Turbellaria

Non-parasitic and aquatic; named for the turbulence their cilia cause in the water.

Monogenea

Flukes that have only a single generation; parasitize fish.

Trematoda

Flukes which have more than one generation, and go through more than one host. The Chinese Liver Fluke starts off in snails, moves to fish, and then attaches itself to the liver of humans who eat the fish.

Cestoidea

The tapeworms. Divided into the scolex, a nightmarish head that attaches itself with suckers or hooks to the host, a neck, and strobila, little packets of eggs that pass in the feces.

Sources:

Campbell & Reece. "Biology" Benjamin Cummings. 6th Edition, 652-654
Miller & Harley. "Zoology" McGraw Hill. 7th Edition, 156-168

* Characteristics are shared by most members of the Phylum; there is some level of individual variation. Characteristics are not noteworthy to the extent that they apply to each member of the Phylum, but to the extent that they differentiate members from the members of other Phyla.