Schwann cells
A chain of supporting cells enclosing the axons of many neurons and forming an insulating
layer called the myelin sheath.
Sclereids
A short, irregular sclerenchyma cell in nutshells and seed coats and scattered through the
parenchyma of some plants.
Sclerenchyma cell
A rigid, supportive plant cell type usually lacking protoplasts and possessing thick secondary
walls strengthened by lignin at maturity.
Scolex
The knoblike anterior end of a tapworm, having suckers or hooklike parts that in the adult
stage serve as organs of attachment to the host on which the tapeworm is parasitic.
Scute
A horny, chitinous, or bony external plate or scale, as on the shell of a turtle or the
underside of a snake. Also called scutum.
Scutellum
A shieldlike bony plate or scale, as on the thorax of some insects.
Sebaceous glands
The small glands in the skin that secrete oil into hair follicles and onto most of the body
surface.
Second law of thermodynamics
The principle whereby every energy transfer or transformation increases the entropy of the
universe. Ordered forms of energy are at least partly converted to heat, and in spontaneous
reactions, the free energy of the system also decreases.
Second messenger
A small, nonprotein, water-soluble molecule or ion, such as calcium ion or cyclic AMP, that
relays a signal to a cell's interior in response to a signal received by a signal receptor protein.
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