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An Overview of Animal Diversity (Concept 3: Animals can be characterized…
An Overview of Animal Diversity
Concept 1: Animals are multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers
Nutrition Mode
Animals get nutrients by either eating other loving organisms or by eating nonliving organic material.
Animals ingest their food.
Cell Structure and Specialization
tissues
: groups of similar cells that act as a functional unit
The cells of most animals are organized into tissues, and perform specific functions, hence
specialization
.
Reproduction and Development
Zygote Development
1) The zygote of an animal undergoes a series of mitotic cell divisions called cleavage.
cleavage
2) An eight-cell embryo is formed by three rounds of cell division.
cleavage
3) In most animals, cleavage produces a multicellular stage called a
blastula
. The blastula is typically a hollow ball of cells that surround a cavity called the
blastocoel.
4) Most animals also undergo
gastrulation
, a process in which one end of the embryo folds inward, expands, and eventually fills the blastocoel, producing layers of embryonic tissues: the ectoderm (outer layer) and the endoderm (inner layer).
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gastrula
: developmental stage in which gastrulation takes place
Organism Development
larva
: sexually immature form of an animal that is morphologically distinct from the adult, usually eats different food, and may even have a different habitat than the adult
metamorphosis
: a development transformation that turns the animal into a juvenile
juvenile
: resembles an adult but is not yet sexually mature
homeoboxes
: sets of DNA sequences that are in regulatory genes
Hox
genes controls the expression of many other genes that influence morphology. It plays important roles in the development of animal embryos.
Concept 2: The history of animals spans more than half a billion years
Prehistoric Timeline
Proterozoic Era
Neoproterozoic Era
(1000-542 MYA)
Paleozoic Era
(251-42 MYA)
Mesozoic Era
(251-65.5 MYA)
Cretaceous Period, Jurassic Period, Triassic Period
Cenozoic Era
(65.5 MYA - Present)
Triassic Period: first
mammals
Jurassic Period: first
birds
Cretaceous Period: first
flowering plants
Cambrian Period
Cambrian explosion
: a relatively brief time in geologic history when many present-day phyla of animals first appeared in the fossil record. This burst of evolutionary change occurred about 535-525 MYA and saw the emergence of the first large, hard- bodied animals.
Hypotheses of Cause of Cambrian Explosion
evolution of
Hox
gene clusters and addition of new microRNAs
natural selection; predator-prey relationship
increase in oxygen content of Earth's atmosphere
Arthropods were the first animals to adapt to terrestrial habitats. Vertebrates colonized land and diversified.
Cambrian Period, Ordovician Period, Silurian Period, Devonian Period
Ediacaran biota
: early group of soft-bodied multicellular eukaryotes
Ediacaran Period
Steps in the Origin of Multicellular Animals
How did animals rise from unicellular organisms?
Multicellularity
requires evolution of new ways for cells to
adhere
and
signal
to each other.
Evolving into Multicellularity
cadherins
: protein that attaches cells to one another
Cadherin attachment protein originated by the arrangement of protein domains found in choanoflagellates plus the incorporation of the CCD domain.
Key steps in transition to multicellularity in animals involved new ways of using proteins or parts of proteins that were encoded by genes found in choanoflagellates.
Concept 3: Animals can be characterized by "body plans"
Symmetry
radial symmetry
: type of symmetry found in a flowerpot, no front or back or right or left sides; example: sea anemones
Radial symmetry allows the animal to meet the environment equally well from all sides.
bilateral symmetry
: type of symmetry with two axes of orientation: front to back and top to bottom; example: lobster
dorsal
- top :
ventral
- bottom :
anterior
- front :
posterior
- back
Most bilateral animals have a CNS that enables them to coordinate complex movements.
Tissues
True tissues are isolated from other tissues by membranous layers and are germ layer tissues during embryonic development. Embryo becomes layered in
germ layers
during gastrulation, forming various tissues and organs of the body.
endoderm
: the innermost germ layer, lines the pouch that forms during gastrulation (the archenteron) and gives rise to the lining of the digestive tract (or cavity) and organs such as the liver and lungs of vertebrates
mesoderm
: germ layer that fills much of the space between the ectoderm and endoderm
ectoderm
: the germ layer covering the surface of the embryo, gives rise to the outer covering of the animal, and in some phyla, to the central nervous system
diploblastic gastrula
: having two germ layers
triploblastic gastrula
: having three germ layers
Triploblasts' mesoderm forms the muscles and most other organs between the digestive tract and outer covering of the animal. Triploblasts include flatworms to arthropods to vertebrates.
Body Cavities
: a fluid-or-air filled space located between the digestive tract and the outer body wall
"true" coelom
: a cavity that forms from tissue derived from mesoderm
pseudocoelomates
: animals with a
pseudocoelom
, a cavity that is formed from mesoderm and endoderm
acoelomates
: animals that lack a body cavity
coelomates
: animals with a true coelom
The Significance of Body Cavities
The fluid cushions suspended organs, helping to prevent internal injury.
The coelom contains noncompressible fluid that acts like a skeleton against which muscles work.
The cavity enables the internal organs to grow and move independently of the outer body wall.
grade
: a group whose members share key biological features
clade
: a group that includes an ancestral species and all of its descendants
Protostome and Deuterostome Development
Cleavage
Spiral and Determinate
spiral cleavage
: planes of cell division are diagonal to the vertical axis of the embryo
determinate cleavage
: casts the development fate of each embryonic cell very early
Radial and Indeterminate
indeterminate cleavage
: each cell produced by early cleavage divisions retains the capacity to develop into a complete embryo
radial cleavage
: planes are either parallel or perpendicular to the vertical axis of the embryo
Coelom Formation
archenteron
: the endodermlined cavity, formed during gastrulation, that develops into the digestive tract of an animal
In deuterostome development, the mesoderm buds from the wall of the archenteron and cavity becomes the coelom.
In protostome development, solid masses of mesoderm split and form the coelom.
Fate of the Blastopore
blastopore: the indentation that during gastrulation leads to the formation of the archenteron
In deuterostome development, the anus develops from the anus.
In protostome development, the mouth generally develops from the blastopore.
protosome development
: in animals, a developmental mode distinguished by the development of the mouth from the blastopore; often also characterized by spiral cleavage and by the body cavity forming when solid masses of mesoderm split
deuterostome development
: in animals, a developmental mode distinguished by the development of the anus from the blastopore; often also characterized by radial cleavage and by the body cavity forming as outpockets of mesodermal tissue
body plan
: a particular set of morphological and developmental traits, integrated into a functional whole - the living animal
Concept 4: Views of animal phylogeny continue to be shaped by new molecular and morphological data
The Diversification of Animals
1. All animals share a common ancestor.
Animals are monophyletic, forming a clade called Metoza.
2. Sponges are basal animals.
Sponges branch from the base of the animal tree and are monophyletic.
3. Eumetazoa ("true tissues") is a clade of animals with true tissues
.
4. Most animal phyla belong to the clade Bilateria.
The members are known as
bilaterians
. The Cambrian explosion was primarily a rapid diversification of bilaterians.
5. There are three major clades of bilaterian animals.
The clades are Deuterostomia, Lophotrochozoa, and Ecdysozoa. Almost every phyla in the clades consist of
invertebrates
(without backbone) except Chordata, which includes
vertebrates
(with backbone).
Ecdysozoa
clade: refers to a characteristic shared by nematodes, arthropods, and some other ecdysozoan phyla. Many ecdysozoans are molting animals. Process of shedding the old exoskeleton is called
ecdysis
.
Lophotrochozoa
clade: Lophotrochozoans include organism that have
lophophores
or
trochophore larvae
.
trochophore larvae
: distinctive larval stage observed in some lophotrochozoan animals, including some annelids and moluscs.
lophophore
: in some lophotrochozoan animals, including brachiopods, a crown of ciliated tentacles that surround the mouth and function in feeding
Deuterostomia
clade: refers to mode of development and members. It contains the Ecdysozoa and Lophotrochozoa clade
Future Directions in Animal Systematics
1. Are sponges monophyletic?
2. Are ctenophores basal metazoans?
3. Are acoelomate flatworms basal bilaterians?
