moves from areas of high-water/low-solute concentra-
tion to areas of low-water/high-solute concentration.
osmotic pressure Pressure that is generated by a
solution moving by osmosis into and out of a cell and
caused by a concentration gradient.
Osteichthyes A class of fish—“bony fish”; Oste-
ichthyes is the largest and most diverse taxon of all ver-
tebrates. Found as early as the lower Devonian period,
there are two subclasses, Actinopterygii (e.g., ray fin:
sturgeon, tuna, catfish) and Sarcopterygii (fleshy fin:
lungfish, coelacanths). Two of the most successful
groups of vertebrates ever known, with some species
adapted to breathe air. Their endoskeleton is made of
bone.
osteoporosis A decrease in bone mass and bone den-
sity with associated increased risk of fracture, especial-
ly of wrists, hips, and spines. Of the 10 million
Americans estimated to have osteoporosis, 8 million
arewomen and 2 million are men.
ostracoderm A primitive and extinct fish without
jaws and encased in an armor of bony plates and der-
mal scales. It lived from the early Ordovician to the late
Devonian (470 to 370 million years ago). Also known
as agnathans, fossils have been found in both North
America and Europe. Pteraspids, cyathaspids, and
amphiaspids are examples.
ostracods Aquatic crustaceans with seven pairs of
appendages, each specialized for different tasks, that
live inside a calcified carapace made of two valves.
Sexually dimorphic, i.e., males and females are differ-
ent shapes. There are more than 50,000 named
species.
outgroup During a phylogenetic analysis, an out-
group is any group that is not included in the study
group but is related to the group under study. Used for
comparative purposes. An outgroup is chosen because
itis related to the taxon under study, but it has an
ancestor in a more distant past than the taxon being
classified.ovarian cycle Determines the menstrual cycle in
the ovary; a regular cyclic event that comprises two
phases and ovulation: the follicular phase (before
ovulation), ovulation itself, and the luteal phase
(after ovulation but before a new follicular phase). It
normally lasts for 11 to 16 days and is regulated by
hormones.ovary The female organ or gonad, located on each
side of the uterus, that produces oocytes that develop
into mature eggs. The ovaries are connected by the
fallopian tubes, also called the oviducts. They also
produce female estrogens (estradiol and proges-
terone) that are responsible for secondary sexual
characteristics.oviduct Another name for fallopian tubes, hollow
organs about 12 centimeters long (about 6 inches) that
join the ovary to the uterus and that transport the
ovum.oviparous Refers to a process in which a female ani-
mal produces eggs that develop and hatch outside the
body. In oviparous species, fertilization occurs when
the sperm meet the ova as they pass through the
oviduct. Insects such as butterflies and moths are
oviparous. They lay their eggs on leaves in which the
larvae hatch.ovotransferrin An iron-binding protein from eggs,
structurally similar to the TRANSFERRINs.ovoviviparous Animals that reproduce by eggs that
remain in the mother’s uterus until they are ready to
hatch. The young emerge alive with only a membrane
to break away from. There is no umbilical cord
attached to a placenta. Some fish and reptiles are
ovoviviparous.256 osmotic pressure