Fitness
The genetic contribution of an individual to succeeding generations relative to the
contributions of other individuals in the population.
Fixed action pattern
A highly stereotypical behavior that is innate and must be carried to completion once
initiated.
Flaccid
Limp; walled cells are flaccid in isotonic surroundings, where there is no tendency for water
to enter.
Flagellum
A long cellular appendage specialized for locomotion, formed from a core of nine outer
doublet microtubules and two inner single microtubules, ensheathed in an extension of
plasma membrane.
Flame cells
organ of excretion in flatworms
Flatworms
Any of various parasitic and nonparasitic worms of the phylum Platyhelminthes, such as a
tapeworm or a planarian, characteristically having a soft, flat, bilaterally symmetrical body
and no body cavity. Also called platyhelminth.
Flower
The reproductive structure of angiosperms; a complete flower includes sepals, petals,
stamens (male structures), and carpels (female structures).
Fluid-feeder
An animal that lives by sucking nutrient-rich fluids from another living organism.
Fluid mosaic model
The currently accepted model of cell membrane structure, which envisions the membrane
as a mosaic of individually inserted protein molecules drifting laterally in a fluid bilayer of
phospholipids.
Follicle
A microscopic structure in the ovary that contains the developing ovum and secretes
estrogens.
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