Platyhelminthes

PLATYHELMINTHES

Lowest of the worms are Platyhelminthes. The have thin soft bodies. This phylum includes three classes:
Turbellaria (free-living flatworms) most of which enhabit fresh water, saltwater or moist places on land.
Trematoda (flukes). External or internal parasites.
Cestoda (Tapeworms) the adults of which are intestinal parasites of vertebrates.
Some of these parasites cause serious illness or death to their hosts.

Characteristics:

1. Symmetry bilateral. Three germ layers (triploblastic). Body usually flattened dorsoventrally. No true segmentation.

2. Epidermis soft and ciliated(turbelaria), or covered with cuticule and with external suckers or hooks, or both for connection to host (Trematoda, Cestoda).

3. Digestive system incomplete (a mouth but no anus) and usually much branched. None in Acoela or Cestoda.

4. Muscle layers well developed. No body cavity. Spaces between internal organs filled by loose parenchyma.

5. No skeletal, ciculatory or respiratory systems. Excretory system with many flame cells connected to excretory ducts (protonephridia).

6. The nervous system is a pair of anterior ganglia or a nerve ring connected to 1 - 3 pairs of longitudinal nerve chords with transverse commissures.

7. The sexes are usually united (monoecious). Reproductive system of each sex with gonads, ducts, and accessory organs. Fertilization occurs internally. The eggs are microscopic, each enclosed with several yolk cells in a shell. The development in its life cycle is either direct (some Turbellaria and monogenetic Trematoda) or with one or more larval stages (digenetic Trematoda and some Turbellaria and Cestoda). Asexual in some species.

Only Turbellaria (free-living flatworms) are dealt with here.

TURBELLARIA

Free-living flat worms

Flatworms have simple, flattened, leaf-like bodies and glide along on a bed of fine hairs or by ripples of contracting muscles.

A marine flatworm with tenacles.

A flexible tubular proboscis traps prey such as small crustaceans and molluscs. The digestive canal ends blindly without any anus.

Cross section of the body at the level of the pharynx.

Lateral view of a generalized tuberlarian.

Flatworms are hermafroditic, but cross fertilise. Partners then lay strings of large yolky eggs.

Prostheceraeus bellostriatus : A marine flatworm.