CHARACTERISTICS OF ANIMALS
- They are heterotrophs
- They are multicellular and lack cell walls
o Some have specialized cells
- Have great mobility thus development of musculature and nervous system
- Reproduce sexually
- Have a characteristic pattern of embryonic development
EVOLUTION OF THE ANIMAL BODY PLAN
Five Key Transitions can be noted in Animal Evolution.
1.
2.
3.
4.
5.
Evolution of Tissues
Different forms of Symmetry
Evolution of a Body Cavity
Different patterns of Development
Evolution of Segmentation
HIERARCHICAL ORGANIZATION OF ANIMAL COMPLEXITY
The following levels of Biological organization are: Cells→ tissues→ organs→ organ systems. In
Animals we see the evolution of tissues.
Cells- Basic building block of life
Tissues- Groups of cells that have the same structural characteristics and perform the same functions
E.g. Epithelial tissue which forms a continuous layer over the entire body surface and
cavities, Connective tissue such as blood, bone and cartilage and Nervous tissue.
Organs- structures composed of two or more types of tissue that work together to perform particular
functions, e.g. skin, heart, intestines etc.)
Systems- composed of various types of tissues and organs to perform certain tasks
SYMMETRY
SPHERICAL- This symmetry is found in protozoa. Any plane passing through the center divides the
body into equivalent mirrored halves, thus the organism has no ends or sides
ASYMMETRICAL- Growing as irregular masses
RADIAL- The parts of the body are arranged around a central axis and divides the organism into halves
that are approximate mirror images. First evolved in Cnidarians.
BILATERAL- A body design in which the body has a right and left half that are mirror images of each
other. The body plan also has a top and a bottom (dorsal and ventral portions), and a
front and back (anterior and posterior) end.
GERM LAYERS
A germ layer is a group of cells, formed during animal embryogenesis. Germ layers are particularly
pronounced in the vertebrates; however, all animals more complex than sponges (eumetazoans and
agnotozoans) produce two or three primary tissue layers (sometimes called primary germ layers).
Animals with radial symmetry, like cnidarians, produce two germ layers (the ectoderm and endoderm)
making them diploblastic. Animals with bilateral symmetry produce a third layer between these two
layers (appropriately called the mesoderm) making them triploblastic. Germ layers eventually give rise to
all of an animal’s tissues and organs through the process of organogenesis.
Ectoderm- forms the skin and nervous system
Endoderm- forms the digestive tract
Mesoderm- produces muscular tissue and parts of the reproductive and circulatory system
Examples Of Our Various Levels Of Organization
1. Protoplasmic- all functions are confined to a cell e.g Unicellular Protists
2. Cellular- aggregation of cells that are functionally differentiated
Examples: Group: Parazoa (Animals that lack specialized tissues)
Phylum: Porifera (Sponges)
Genus: Genus- Grantia
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Very simple morphology but they are highly adaptive and have survived for thousands of years
Their morphology allows for acquiring nutrients and reproduction
They are eukaryotic, multicellular and heterotrophic
Their body plan is asymmetrical (no symmetry or pattern, i.e no left or right halves etc.)
They do not have distinct tissues or organs instead are just made up of aggregates of cells that can
differentiate and de-differentiate
o Therefore the cells can separate from each other and can gather back together again to
form a functioning organism
o Each disassociated cell can also survive on its own
Grantia have tubular bodies with open ended chambers, and possess pores for filtering H20
http://www.ips.it/scuola/concorso/ambiente/grantia.jpg
3. Cell-Tissue – cells are aggregated into patterns/layers forming tissues
Example: Group: Radiata (Animals that have radial symmetry)
Phylum: Cnidaria (Sea Anemones)
Genus: Genus- Metridium
- Cnidarians are marine carnivores and include hydras, jellyfish, anemones and corals
- exhibit radial symmetry and thus are more complex than sponges
- Cnidarians possess ‘true tissues’ which include rudimentary nervous, muscular, and reproductive
tissues, but no organs
- Metridium is a type of sea anemone which is characterized by numerous tentacles that extend from
atop a smooth cylindrical base, and can vary from a few centimeters in height or up to several meters
http://rydberg.biology.colostate.edu/Dissections/metridium.htm
http://homepage.uab.edu/acnnnghm/BY255L/BY255LImages/BY255LImages-Cnidaria/Metridium-2.jpg
Gonads
Jellyfish
4. Tissue- Organ – cells are aggregated into patterns/layers forming tissues
Example: Group: Bilateria (Animals that have bilateral symmetry)
Phylum: Platyhelminthes (Flatworms)
Genus: Genus- Dugesia (Planarian)
- Platyhelminthes occur in marine, freshwater, terrestrial and parasitic environments
- they have a cellular mesoderm in addition to an ectoderm and endoderm
- Dugesia (planaria) has lateral lobes and sensory organs called eyespots in addition to a rudimentary
digestive system and a pharynx
5. Organ-System – organs work together to perform a coordinated function
Example: Group: Bilateria (Animals that have bilateral symmetry)
Phylum: Chordata (Chordates, such as fish, amphibians and reptiles)
Genus: Genus- Perca (Perch)
- The perch belongs to Class Osteichthyes (Bony Fish) whichmeans they have a bony endoskeleton,
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modified gill arches.
The perch organ systems include a digestive, circulatory, urinary and reproductive system
BODY CAVITIES
The evolution of the body cavity in bilateral symmetrical organisms facilitated the support and
housing of organs, along with aiding distribution of materials in the body and specific embryonic
development patterns.
o Thus the development of advanced organ systems
A body cavity refers to a space surrounded by mesodermal tissue
There are three types of body plans, Acoelomate, Pseudocoelomate and Coelomate.
1. Acoelomates
- Have no body cavity between the mesoderm and the endoderm, instead there is a solid mass of
tissue composed of cells and organic material
Example: Group: Bilateria (Animals that have bilateral symmetry)
Phylum: Platyhelminthes (Flatworms)
Genus: Genus- Dugesia (Planarian)
- Planaria have no body cavity besides their digestive cavity, andit allows for regeneration
2. Pseudocoelomates
- Possess a body cavity called a pseudocoel that is found between the mesoderm and the endoderm
Example: Group: Bilateria (Animals that have bilateral symmetry)
Phylum: Nematoda (Roundworms)
Genus: Genus- Ascaris
- Ascaris is a parasitic roundworm that infects the intestinal tract of many vertebrates including humans
- Male Ascaris are usually smaller than the females, and posses a hooked posterior end
3. Coelomates
- Possess a fluid-filled body cavity entirely within the mesoderm called a coelom.
- Many organ systems are suspended in the coelom which is surrounded by a layer of epithelial cells
Example: Group: Bilateria (Animals that have bilateral symmetry)
Phylum: Annelidia (Segmented Worms e.g earthworms and leeches)
Genus: Genus- Lumbricus (earthworms)
- The Phylum Annelida is defined by the division of the body into repetitive segments
o Each segment contains parts of the circulatory, digestive, nervous and excretory systems
BILATERIANS AND DEVELOPMENT
Bilaterians can be further divided into two groups according to their basic pattern of development. The
two groups are Protosomes and Deuterosomes.
1.
Protosomes
- The mouth develops before the anus
- Some examples of protosomes includes flatworms, nematodes and mollusks
- Protosomes also display Determinate development
o Determinate development results in the developmental fate of the cells being set early in
the embryo development. Each cell produced by early embryonic cleavage does not have
the capacity to develop into a complete embryo.
2. Deuterosomes
- The anus develops before the mouth
- Some examples of deuterosomes include the Phylum Echinodermata (sea stars, urchins, sea
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cucumbers), and the Phylum Chordata ( fish, reptiles, birds, mammals,and amphibians)
Deuterosomes display Indeterminate dvelopment
o The fate of the cell is not predetermined, thus when the original cell in a deuterostome
embryo divides, the two resulting cells can be separated, and each one can individually
develop into a whole organism.
Describing Positions
Dorsal- toward the upper surface (back)
Ventral – toward the lower surface (belly)
Anterior - toward the head
Posterior- toward the tail
Cranial- toward the head
Caudal- toward the tail
Medial- toward the midline of the body
Proximal- toward the end of the appendage nearest the body
Lateral- away from the midline of the body
Distal-toward the end of an appendage farthest away from the body
Frontal Plane- divides the body into dorsal and ventral halves
Transverse Plane- a cross section perpendicular to the anterior-posterior axis
Saggital Plane- divides the body into left and right halves
Bibliography
Vodopich D, Moore R.2008. Biology Laboratory Manual Eighth Edition. McGraw Hill, NY,New York.
Raven P, Johnson G, Losos J, Mason K, Singer S. BiologyEight Edition.McGraw Hill, NY New York.