Anterior Sections            Posterior Sections           Flame Cell Close-Up

This model includes several views of a free-living turbellarian flatworm. The image on the left shows the nervous system (painted white), which consists of a pair of cerebral ganglia with two ventral nerve cords that are connected by a series of transverse nerves called commissures, giving it a ladder-like appearance. Other sensory structures include simple, light-sensitive eyes (ocelli) and chemical receptors that are concentrated in lateral projections of the head called auricles (because they look like ear lobes). Although reproduction in planarians can occur asexually through fission, all forms are monoecious with both male and female reproductive organs. Several features of the reproductive system (shown in blue and yellow) are also seen on the model on the left. 

The model on the right shows the many branched gastrovascular cavity (shown in red) that exits through a single ventral opening at the end of a muscular, eversible pharynx (shown in off-white on both models as well as on the small upper planarian
model).  Also seen on the model is a portion of the excretory/osmoregulatory system (shown in green) that is made up of protonephridia that collect and secrete some wastes as well as the excess water that enters freshwater forms by osmosis. Protonephridia consist of excretory tubules that are closed internally and open to the outside by a series of collecting ducts that lead to a posterior opening called a nephridiopore. The internal ends of each of these tubules terminate in so-called flame cells (one of which is shown on the small, lower model), which have tufts of cilia that flicker like the flame of a candle. The beating of these cilia pulls water through a mesh-like cup, producing a filtrate of water and small molecules.