Nerve activates contraction

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A brief overview of where we are going
-Animals are multicellular, ingestive
eukaryotes -Many eat dead material, but most eat
other things (plants, protists, fungi, animals,
etc.) that are alive. -First animals fed
like protozoans, with no mouth, no gut. Took
small pieces of food directly into feeding cells,
using phagocytosis.
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A brief overview of where we are going
-But for a multicellular ingestive heterotroph,
the ability to -search for food -chase and
capture elusive prey -eat things bigger than one
cell -and evade animals that would eat it would
provide ways to make a living (and keep
living) The major trends in animal evolution,
then, have been related to -locomotion -body
size -methods of food gathering and
digestion -senses -and all the organ systems
to that support activity and large body size
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A brief overview of animals
-Complexities of animal structure are achieved by
modifications of the patterns of embryonic
development -Animals share basic aspects of
early development, and modifications of some
features of early development are reflected in
basic differences in body plans -The evolution
of these body plans will be the main theme in our
study of animals
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Animal Development
-How are complex animals assembled from one
undifferentiated ovum? One idea Preformation
Another idea Epigenesis embryo gradually
takes shape
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Animal Development
-What we will discuss -Patterns of early
development -How the large zygote becomes
organized into the basic components of the
organism to be -Variations in this process,
and how that is related to phylogeny -Mechanisms
controlling the development of organs and organ
systems
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Early development
-Three stages, after fertilization -Cleavage
the orderly division of the zygote into smaller
cells, which can move around, and which contain
different complements of chemicals laid down in
the ovum -Formation of the blastula, a stage
that is found, in one form or another, in
essentially every animal embryo -Gastrulation
Invagination of the blastula, producing the basic
organization of the embryo to be
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Early Embryonic Development
Cleavage -Orderly cell divisions -Simultaneous
mitoses -Little cell growth -Little activity by
genes in the nucleus!
Animal Pole Vegetal Pole (the end with the
mot yolk)
Each cell a Blastomere
Gastrulation -Invagination of the blastula,
producing an embryo with at least two layers, and
an opening from the outside to the inside.
Blastula Stage -Small cells, which can be moved
around. -Different kinds and amounts of material
from cytoplasm of egg
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 Sea urchin gastrulation
Archenteron Future gut Blastopore Mouth or
anus (or both), depending on the kind of
animal Blastocoel The space inside the
blastula. It largely disappears in many animals,
but persists in some.
Gastrulation provides -Orientation of embryo
-Inside and outside -Front
and back -Basic layers of cells that will develop
into tissues
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Variations in early development -Effects of yolk
on cleavage (big, yolky eggs dont cleave
completely)
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Spiral vs. Radial Cleavage
Plane of cleavage
Spiral cleavage Cleavage plane is oblique
Radial cleavage Cleavage plane is at right angle
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Determinate Cleavage Ultimate fate of
blastomeres is determined from the very first
cleavage
Indeterminate Cleavage For a while, at least,
the fate of blastomeres is flexible
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But even indeterminate cleavage has limits.
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Number of germ layers In some animals, only two
germ layers appear (the endoderm and ectoderm).
Animals of the phylum Cnidaria (jellyfish, sea
anemones, and corals) are an example of this. In
most other familiar animals a mesoderm layer
develops as well, producing three germ layers and
some of the additional tissues that develop from
mesoderm. Number of openings to the gut In some
animals, like Cnidarians and Platyhelminthes
(flatworms), there is only one opening to the
gut, and it derives directly from the
blastopore. In most other animals, a second
opening develops at the other end of the
archenteron from the gut, providing a complete
(mouth to anus) gut. More on the evolutionary
significance of these features later on.
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What happens next? Morphogenesis (development of
form) and Organogenesis (development of organs
and organ systems)
Body organization and organ systems unfold
gradually in an embryo. In this frog embryo, the
basic orientation of the embryo appears at the
end of gastrulation. -Dorsal and ventral
sides -Anterior and posterior ends -Gut cavity to
be -Germ layers
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Organogenesis in a frog embryo the gradual
differentiation of organ systems, followed by
development of specific organs within the organ
systems
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Later stages in the development of an embryo
Brain, ear capsule, eye development of the
neural tube
Developing gill slits (from digestive tube)
Limb bud (from mesoderm)
Somites (segments from mesoderm, to form body
muscles and vertebrae)