Turbellaria, free-living flatworms
Turbellaria include the only
free-living Platyhelminthes, they are mainly aquatic, but there are a few
terrestrial species inhabiting moist environments. A small number of terrestrial species have become pests where they have been unintentionally introduced in pot plants (see the New Zealand flatworm below). There are over 4000 species living on rock and sediments on water and in moist habitats on land, and they range in size from 1 mm - 50 cm.
Locomotion. They usually have ventral
cilia, and this, along with mucous from gland cells forming a trail, is their
means of locomotion. The larger species also rely on muscular contractions, and
ventral cilia may be absent in these species.
The mouth is usually located on the underside of the body, part of the way back from the front edge (see right), and can face forwards or backwards depending on the body type.
Eyes or light-sensitive eye spots are fairly
common and are located at the head end. Some have statocysts for equilibrium.
Reproduction. Turbellarians can reproduce asexually
by fission, and whole animals can regenerate from a small piece, with the
original piece retaining polarity.
There are two basic body types as shown in the drawing on the right - Triclads on the left and Polyclads on the right.
Triclads, on the left in the drawing opposite, have 3 main branches to the gut: 1 pointing forwards, and 2 pointing backwards. They include the freshwater planarians. Planarians are mostly carnivorous, and feed on Crustacea, Nematoda, Rotifera and Insecta. Freshwater triclads often rest on the undersides of stones, leaves or among mosses. They are sometimes mistaken for leeches. They can also travel upside down along the surface of water, their weight being supported by surface tension. In pale coloured species they will appear to be the colour of what they have most recently eaten. They lay their eggs in cocoons attached to stones or water plants.