sponges; animals with cells that form tissues and
organs, a mouth, and digestive tract. Two branches
exist, the radiata and the bilateria. The radiata have
radial symmetry, i.e., all longitudinal planes are equal
around a central body axis, while the bilateria
are animals that have bilateral symmetry, i.e., they
have a definite front and rear, and left and right body
surfaces.
Eurasia Europe plus Asia considered as one conti-
nent. Used in political, economic, and geographical
terms.
eusocial A social system of insects, belonging to the
order Isoptera (termites) and the order Hymenoptera
(ants, bees, and wasps), in which the individuals coop-
erate in caring for the young after one female produces
offspring. There is a reproductive division of labor, and
previous generations aid in rearing. This trait also
occurs in two species of mammals (mole rats).
eutherian mammals(placental mammals) The female
has a placenta that is connected to an embryo within
the uterusthat supplies it with nutrients and oxygen
and acts as an excretory system. Humans are eutherian
mammals.
eutomer The enantiomer of a chiral compound that
is the more potent for a particular action.
See alsoDISTOMER.
eutrophication The accelerated loading or dumping
of nutrients in a lake by natural or human-induced
causes. Natural eutrophication changes the character of
a lake very gradually, sometimes taking centuries, but
humanmade or cultural eutrophication speeds up the
aging of a lake, changing its qualities quickly, often in a
matter of years.
eutrophic lake Any lake that has an excessive supply
of nutrients, usually nitrates and phosphates. Eutrophic
lakes are usually not deep, contain abundant algae or
rooted plants, and contain limited oxygen in the bot-
tom layer of water.
See alsoMESOTROPHIC LAKE;OLIGOTROPHIC LAKE.evaporative cooling Temperature reduction when
water absorbs latent heat from the surrounding air as it
evaporates. Similarly, “cooling” of the skin from the
evaporation of sweat is evaporative cooling and is a
process for the body to lose excess heat.evolution The long process of change that occurs in
populations of organisms. It began with the first life
forms on Earth and created the diversity of life forms
that exist today and that will exist in the future.evolutionary species concept Aspecies comprises
the totality of individuals that share acommon evolu-
tionary history. A species is a lineage evolving separate-
ly from others.evolve Change slowly.Ewens–Bassett number SeeOXIDATION NUMBER.EXAFS See EXTENDED X-RAY ABSORPTION FINE
STRUCTURE.exaptation The adoption of an attribute that had
one function in an ancestral form but now has a new
and different form, e.g., swim bladders becoming
lungs, or three jaw bones of mammal ancestors becom-
ing the middle bones of the ear. Formerly called
preadaptation.excitatory postsynaptic potential (EPSP) Electri-
cal change in the membrane of a postsynaptic neuron
caused by binding of an excitatory neurotransmitter
from a presynaptic cell to a postsynaptic receptor. Pro-
motes firing of an action potential in the postsynaptic
cell.120 Eurasia