AP Biology
Notes: Introduction to Animals
·
Animal life began in
Precambrian seas with the evolution
of multicellular forms that lived by eating other organisms.
·
Early animals populated
the seas, fresh waters, and
eventually the land.
Animal
characteristics
Structure, nutrition and life history define animals
While there are exceptions to
nearly every criterion for distinguishing
an animal from other life forms, five criteria, when taken
together,
create a reasonable definition.
1)
Animals are multicellular,
heterotrophic eukaryotes.
·
They must take in
preformed organic molecules through ingestion,
eating other organisms or organic material that is decomposing.
2) Animal cells lack cell walls that provide structural supports for plants and fungi.
·
The multicellular bodies
of animals are held together with the
extracellular proteins, especially collagen.
·
In addition, other
structural proteins create several types of intercellular
junctions, including tight junctions, desmosomes, and gap
junctions,
that hold tissues together.
3) Animals have two unique types of
tissues: nervous tissue for impulse
conduction and muscle tissue for movement.
4)
Most animals reproduce sexually, with the diploid stage usually dominating the
life cycle.
·
In most species, a small
flagellated sperm fertilizes a larger, nonmotile eggs.
·
The zygote undergoes cleavage,
a succession mitotic cell divisions, leading
to the formation of a multicellular, hollow ball of cells called the blastula.
·
During gastrulation,
part of the embryo folds inward, forming the
blind pouch characteristic of the gastrula.
This produces two tissue layers:
the endoderm as the inner layer and the ectoderm as the
outer layer.
·
Some animals develop
directly through transient stages into adults,
but others have distinct larval stages.
that is morphologically distinct from the adult, usually eats different
foods, and
may live in a different habitat from the adult.
· Animal larvae eventually undergo metamorphosis, transforming the animal into an adult.
5)
The transformation of a zygote to an animal of specific form
depends on the controlled
expression in the developing embryo of special regulatory genes called Hox genes.
·
These genes regulate the
expression of other genes.
·
Many of these Hox
genes contain common “modules” of DNA sequences,
called homeoboxes.
·
Only animals possess
genes that are both homeobox-containing in structure and
homeotic in function.
·
All animals, from sponges
to the most complex insects and vertebrates have
Hox
genes, with the number of Hox genes
correlated with the complexity of
the animal’s anatomy.
Evolution of animal Kingdon
·
Animal kingdom is monophyletic.
they would converge on a common ancestor.
·
That ancestor was most
likely a colonial flagellated protist that lived over 700 million years
ago in the Precambrian era.
·
This protist was probably
related to choanoflagellates, a group that arose about a
billion years ago.
ponds, lakes, and marine environments.
Animal
diversity:
There are three main
hypotheses for what caused the diversification of animals.
1) Ecological Causes: The emergence of predator-prey relationships led
to a diversity of evolutionary adaptations, such as various kinds of protective
shells and diverse modes of locomotion.
2) Geological Causes: Atmospheric oxygen may have finally reached high
enough concentrations to support more active metabolism.
3) Genetic causes: Much of the diversity in body form among animal
phyla is associated with variations in the spatial and temporal expression of Hox
genes within the embryo.
·
A reasonable hypothesis
is that the diversification of animals was associated
with the evolution of the Hox
regulatory genes, which led to variation in morphology
during development.
1)
The first branch point splits the Parazoa
which lack true tissues
from the Eumetazoa which have true
tissues.
·
The parazoans, phylum
Porifera or sponges, represent an early branch of the animal kingdom.
·
Sponges have unique
development and a structural simplicity.
2)
The
eumetazoans are divided into two major branches,
partly based on body symmetry.
·
Members of the phylum
Cnidaria (hydras, jellies, sea anemones and their relatives)
and phylum
Ctenophora (comb jellies) have radial
symmetry and are known
collectively as the radiata.
·
The other major branch,
the bilateria, has bilateral
symmetry with a dorsal
and ventral side, an anterior
and posterior end, and a left and
right side.
·
Linked with bilateral
symmetry is cephalization, an
evolutionary trend toward the
concentration of sensory equipment on the anterior
end.
development of a central nervous system concentrated in the head
and extending toward
the tail as a longitudinal nerve cord.
The symmetry of an animal
generally fits its lifestyle.
·
Many radial animals are
sessile or planktonic and need to meet the environment
equally well from all
sides.
·
Animals that move
actively are bilateral, such that the head end is usually first to
encounter
food, danger, and other stimuli.
The basic organization of
germ layers, concentric layers of
embryonic tissue that form various
tissues and organs, differs between radiata
and bilateria.
·
The radiata are said to
be diploblastic because they have two
germ layers.
·
The ectoderm,
covering the surface of the embryo, gives rise to the outer covering
and, in
some phyla, the central nervous system.
·
The endoderm,
the innermost layer, lines the developing digestive tube, or
archenteron,
and gives rise to the lining of the digestive tract and the organs derived from
it,
such as the liver and lungs of vertebrates.
·
The bilateria are triploblastic.
·
The third germ layer, the
mesoderm lies between the endoderm
and ectoderm.
·
The mesoderm develops
into the muscles and most other organs between the
digestive tube and the outer
covering of the animal.
3)
The Bilateria can be divided by the presence or absence of a body
cavity (a fluid-filled space
separating the digestive tract from the outer
body wall) and by the structure the body cavity.
·
Acoelomates
(the phylum Platyhelminthes) have a solid body and lack a body cavity.
completely lined by mesoderm.
·
These pseudocoelomates
include the rotifers (phylum Rotifera) and the roundworms
(phylum Nematoda).
·
Coelomates
are organisms with a true coelom, a
fluid-filled body cavity completely
lined by mesoderm.
·
The inner and outer
layers of tissue that surround the cavity connect dorsally and
ventrally to form
mesenteries, which suspend the internal organs.
·
A body cavity has many
functions.
·
Its fluid cushions the
internal organs, helping to prevent internal injury.
·
The noncompressible fluid
of the body cavity can function as a hydrostatic
skeleton against which muscles
can work.
·
The presence of the
cavity enables the internal organs to grow and move
independently of the outer
body wall.
(4)
The coelomate phyla are divided into two grades based on differences in their
development.
·
The mollusks, annelids,
arthropods, and several other
phyla belong to the protostomes,
while echinoderms,
chordates, and some other phyla belong to the deuterostomes.
These differences center
on cleavage pattern, coelom formation, and blastopore fate.
Cleavage:
·
Many protostomes undergo spiral
cleavage, in which planes of cell division
are diagonal to the vertical axis
of the embryo.
·
Some protostomes also
show determinate cleavage where the
fate of each
embryonic cell is determined early in development.
·
The zygotes of many
deuterostomes undergo radial cleavage
in which the
cleavage planes are parallel or perpendicular to the vertical egg
axis.
·
Most deuterostomes show indeterminate
cleavage whereby each cell in the
early embryo retains the capacity to
develop into a complete embryo.
Coelom formation:
·
As the archenteron forms
in a protostome, solid masses of mesoderm split to form the coelomic cavities,
called schizocoelous development.
·
In deuterostomes,
mesoderm buds off from the wall of the archenteron and hollows to become the
coelomic cavities, called enterocoelous
development.
Blastopore formation (the opening of the archenteron):
·
In many protosomes, the
blastopore develops into the mouth and a second opening at the opposite end of
the gastrula develops into the anus.
·
In deuterostomes, the
blastopore usually develops into the anus and the mouth is derived from the
secondary opening.