Organ system
intergrated group of organs, performing one or more unified functions.
Tissue
Association of cells with a common embryological origin or pathway and similar structure and function. Usually, cells of a tissue are contiguous at cell membranes and may be of one or more types. Tissues aggregate to form organs.
epithelial cell
cell of epithelium, the layer of cells that cover the external surface of an organism as well as forming an inner lining between the tissues and organs of that organism and the external environment. In C. elegans epithelial cells include epithelium of the alimentary tract, intestinal cells, hypodermis, interfacial cells and somatic gonad cells.