Platyhelminthes: the building block for other invertebrates
Simpler animals than those first found in the fossil records still inhabit the earth and its oceans and their ancestors may have represented the predecessors for the shelled invertebrates that are found in the fossil records. These soft-bodied animals belong to the phylumPlatyhelminthes. The most basic of these animals is the flatworm, a flat-leaf shaped worm which like jellyfish have a single opening to their gut through which food is ingested and waste is ejected. Their bodies have differentiated into three layers, the ectoderm, mesoderm and endoderm. Cells with a different structure and function have aggregated to form a primitive system (e.g. nervous system which consists of a network of nerve fibres). Nevertheless, they have no breathing system with oxygen diffusing directly through the skin. Their undersides are covered with cilia which, by beating, permits them to glide over surfaces. Their front end has a mouth on the under-surface and a few light sensitive spots above.