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Review Sheet Ch. 32 Intro to Animal Evolution
1) Characteristics of Kingdom Animalia: multicellular, chemoheterotrophic,
method of taking in
nutrients is ingestion (in contrast to fungi, for example, where nutrients
are absorbed);
2) no cell walls
3) most have nerves and muscles, other taxonomic groups lack these tissues
4) most reproduce sexually (large non-motile egg; small flagellated sperm);
Zygote divides by mitosis
(cleavage) into a blastula (a hollow ball of cells). One end of the blastula
folds inward in a process
called gastrulation, forming a blind pouch called the archenteron. After
gastrulation the blastula is
called a gastrula The opening to the archenteron is called the blastopore.
The archenteron becomes
the digestive cavity in most animals.
5) Animals are found in most habitats on earth. Most phyla are found in the
ocean. On land, vertebrates
and insects are dominant.
6) The traditional taxonomic groupings of animals are as follows:
Animalia
Parazoa
Eumetazoa
Radiata
Bilaterata
Acoelomates
Animals with body cavities
Pseudocoelomates
Coelomates
Protostomia
Deuterostomia
7) A newer model based on molecular data (which we will emphasize over the
traditional model):
Animalia
Parazoa
Eumetazoa
Radiata
Bilateria
Deuterostomia
Protostomia
In this newer model, the acoelomate and pseudocoelomate conditions evolved
secondarily in Protostomes
that had true coeloms.
8) Parazoa have no true tissues, organs or nerves. Sponges (Porifora) are the
only phylum in this group.
All other animals are the Eumetazoa group.
9) Radiata all have radial symmetry. This means they have no head or tail
end. They do have a top and
bottom (the oral and aboral ends). The main groups of Radiata are jellyfish
and hydras (Phylum
Cnidaria) and comb jellies (Phylum Ctenophora). Radiata are also
diploblastic. This means they lack
a mesoderm during embyonic development. The mesoderm gives rise to muscles
and most other
organs besides the skin, nerves, and gut. Radiata do have an ectoderm
(develops into skin and nerves)
and an endoderm (develops into gut). That means Radiata has skin, nerves and
a gut, but not most of
the other organs found in Bilateria.
10) Bilateria have bilateral symmetry. This means they have a head, a tail,
and distinct right and left
sides. Some Bilateria have radial symmetry as adults (e.g. sea stars, sea
cucumbers), but all are
bilateral at some point during early development. Bilateria are
triploblastic: they have endoderm,
ectoderm, and they also have a mesoderm and the tissues that develop from the
mesoderm.
11) Bilateral symmetry is an adaptation for an active, mobile lifestyle,
while radial symmetry is more
suited for a sessile lifestyle. Bilateral symmetry is associated with
cephalization, which is a
concentration of sensory organs at one end of the animal (forming a head).
12) Bilateria can be divided into two major taxonomic groups: protostomes and
Deuterostomia.
13) Protostome means “mouth first”. This means that the blastopore develops
into the mouth. In
protostomes with complete, one-way guts (i.e. a mouth and an anus), a second
infolding of the gastrula
pushes in until it meets the archenteron, forming a complete tube (the
digestive tract). At this point the
embryo is shaped like a donut. The second pore becomes the anus. See fig32.7.
14) Deuterostome means “mouth second.” This means that the blastopore does
not develop into the
mouth. The blastopore of deuterstomes develops into the anus. While the
second pore that develops
following gastrulation becomes the mouth.
15) Other key difference between protostomes and deuterostomes:
Protostome
Deuterostome
Spiral cleavage (planes of cell division are diagonal Radial cleavage (planes
of cell division parallel or
to vertical axis of embryo)
perpindicular to vertical axis of embryo)
Determinate cleavage (developmental fate of
Indeterminate cleavage (developmental fate of
individual embryonic cells is set; i.e. no twins)
individual embyonic cells is not set; i.e. a split
embryo may develop into twins)
16) The coelom (“sea-lum”) is the body cavity, a fluid filled space
separating the gut from the outer body
wall. This is where our viscera (heart, lungs, kidneys), muscles, etc. are
located. Some bilateria, like
the flatworms, are acoelomates. They have a solid body with no cavity between
their gut and outer
body wall. Organisms with body cavities are divided into coelomates and
pseudocoelomates. In
coelomates, the body cavity is completely lined with mesoderm. In
pseudocoelomates, the cavity is
not completely lined with mesoderm. The main example of pseudocoelomates are
the roundworms.
Coelomates include chordates, echinoderms, arthropods, mollusks, annelids and
other phyla.
17) A large question in animal phylogenetics is whether the acoelomate phyla
(e.g. flatworms) and the
pseudocoelomate phyla (e.g. roundworms) evolved from non-coelomate or
coelomate ancestors. For
example, it was originally believed that the flatworm’s lack of body cavity
was a primitive condition.
However, recent evidence suggests that flatworm’s descend from coelomate
ancestors, and evolved to
a simpler body plan. This would mean that the flatworm’s lack of body cavity
is a derived condition.
18) Origins of animal phyla: The animal phyla that exist today did not appear
gradually in the fossil record
over geological time, rather they all originated during a ~40 million year
period. This was about 565
to 525 million years ago. This period is called the Cambrian explosion,
because so many new phyla
suddenly appeared.
19) The history of life on Earth is divided into four eras. The first is also
the longest and is called the
Precambrian era. It starts with the origin of the Earth, and extends to the
time when animals first
began leaving easily recognizable fossils, from shells and other hard body
parts (the beginning of the
Cambrian era). The time when Kingdom Animalia originated is uncertain, but it
was definitely in the
Precambrian era (possibly at the very end, but also possibly a billion years
ago or earlier).
20) Why did the animal phyla evolve so rapidly? Possible explanations: a)
ecological: predator prey
relationships caused coevolution, biological arms races that promote rapid
change;
b)
environmental: it is possible the concentration of O2 in the atmosphere
increased to a level sufficient to
sustain the more active metabolisms of mobile animals; c) Genetic: the
appearance of Hox genes
(genes that regulate embryonic development) may have opened the door for many
different
combinations of major genotypes resulting in different major body plans (e.g.
phyla).
21) Why have no new phyla appeared since the Cambrian explosion? It appears
that by the end of the
Cambrian explosion, the animal phyla had become fixed into development
patterns such that offspring
with major changes in developmental characteristics or body plan would be
strongly selected against
(i.e. not be able to survive and or reproduce).
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INVERTEBRATES
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