Sphingosine
A basic, long chain, unsaturated amino alcohol, C 18 H 37 NO 2 , found combined with lipids in
the brain and in tissue.
Sphincter
A ringlike valve, consisting of modified muscles in a muscular tube, such as a digestive tract;
closes off the tube like a drawstring.
Spicule
A needlelike structure or part, such as one of the mineral structures supporting the soft
tissue of certain invertibrates, especially sponges.
Spinal cord
Part of the vertebrate central nervous system; consists of a thick, dorsal, longitudinal bundle
of nerve fibers extending posteriorly from the brain.
Spindle
An assemblage of microtubules that orchestrates chromosome movement during eukaryotic
cell division.
Spine
Any of various pointed projections, processes, or appendages of animals.
Spiracle
One of the external openings of the respiratory system in terrestrial arthropods.
Spiral cleavage
A type of embryonic development in protostomes, in which the planes of cell division that
transform the zygote into a ball of cells occur obliquely to the polar axis, resulting in cells of
each tier sitting in the grooves between cells of adjacent tiers.
Spirilla
Any of various aerobic bacteria of the genus Spirillum, having an elongated spiral form and
bearing a tuft of flagella.
Spleen
A large, highly vascular lymphoid organ, lying in the human body to the left of the stomach
below the diaphragm, serving to store blood, disintegrate old blood cells, filter foreign
substances from the blood, and produce lymphocytes.
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