|
Flat (up to 0.5mm in its longest dimension), multicellular, amorphous, mobile flagellated animal lacking body cavity, digestive and nervous systems, composed of 2 layers of epithelial cells. Associated glandular cells apparently secrete digestive enzymes beneath the animal as it sits atop the algae and protozoans on which it apparently feeds; digestion seems to be entirely extracellular , as there is no mouth and no sign of phagocytosis. The much thinner, upper layer of the animal bears flagellated cells, but no gland cells. In a sense, the upper layer is ectodermal, while the lower layer, because of its involvement in digesting food and absorbing nutrients, is endodermal. Between the upper and lower cell layers is a fluid-filled space containing a dense network of fibrous cells that may be contractile (Ref. 53). |