[Solved] "Money" provides a ratio scale. Endoderm: The innermost of the three primary germ layers in the embryo, giving rise to the lining of the digestive and respiratory systems. Digestive Tube: A continuous hollow tube extending from the mouth to the anus, through which food is processed and nutrients absorbed. Plankton: Free-floating, mainly microscopic aquatic organisms found in the upper layers of the water; consisting of phytoplankton and zooplankton. Compare with nekton. Microscopic Organisms: Organisms too small to be seen with the naked eye, often including bacteria, archaea, some fungi, and protists.