Receptor: An organ specialized to receive stimuli, e.g., a sense-organ.
Sedimentary: A term applied to rocks which originated as sediments deposited under water.
Serum: Watery portion of the blood, the plasma.
Somatic Cells: Vegetative cells not especially set aside by the organism for reproductive purposes, e.g., tissue-cells.
Somite: One of the uniform segments of the longitudinal series into which a metameric organism (such as an earthworm) is partitioned.
Spermatist: An old term applied to one who held that the animal embryo was produced entirely by the male parent.
Spore: A single cell, incapable of syngamy, but capable of giving rise to a new individual without the sexual process.
Symbiosis: The obligatory association of two organisms of different species for mutual benefit.
Synapsis: Union in pairs of corresponding (homologous) chromosomes of opposite parental origin as a preliminary to their separation in meiosis.
Systematist: An expert in classification (systematics), i. e., a taxonomist.