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Chapter 3

BIOL 2030 Chapter Notes - Chapter 3: Mollusca, Lophotrochozoa, Sagittal Plane

Course Code
BIOL 2030
German Reyes

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Chapter 3: Animal Architecture
The Hierarchical Organization of Animal Complexity
Eukaryotic cell is the basic building block of animals
There are 5 major levels of organization within the eukaryotes
1. Protoplasmic Level
a. Characterizes unicellular organisms
b. All life functions confined within the cell
i. Organelles perform specialized functions
c. Cells vary in terms of their subcellular structures
2. Cellular Level
a. Either colonial or multicellular
i. Colonial
1. Aggregation of undifferentiated cells
2. Unspecialized
3. All are able to reproduce
ii. Multicellular
1. Aggregation of functionally differentiated cells
2. Specialized
3. Reproduction is limited to certain cells only
4. Its evolution evolved independently in as many as 25 lineages of which is a lineage, called
Opisthokonta, which led to the animals
b. The aggregated cells communicate with each other
i. May secrete extracellular matrix (ECM) to which cells adhere
c. Examples: protozoan colonial forms and some place the sponges (phylum: porifera) at this level
3. Cell-Tissue Level
a. Similar cells aggregate into definite patterns or layers
b. Tissue = a group of similar cells organized to perform a common function
c. Has 3 distinct types of junctions between cells:
i. Adherens
ii. Separate
iii. Gap junctions
d. Examples: Cnidarians such as jellyfish; some place sponges at this level
i. Excellent example of tissue is the Cnidarian nerve net formed from nerve cells and function in
4. Tissue-Organ Level
a. Tissues assemble into organs
b. More specialized function than tissues
c. Usually one type of tissue carries the burden of an organ’s chief function
i. Parenchyma = chief functional cells of an organ
1. E.g. muscle tissue in heart
ii. Stroma = supportive tissues of an organ
1. E.g. epithelial, connective, and nervous tissues in heart
d. Example: flatworms (platyhelminthes)
i. Flatworms have well-defined organs such as eyespots, a digestive tract, and reproductive organs
5. Organ-System Level
a. Organs operate together to perform some function
b. Systems are associated with basic body functions
c. 11 Systems in metazoans:
i. skeletal
ii. muscular
iii. integumentary
iv. digestive
v. respiratory
vi. circulatory
vii. excretory
viii. nervous
ix. endocrine
x. immune
xi. reproductive
d. Examples: most animals demonstrate this level of organization
i. Some examples are: frog (phylum: chordate), crab (phylum: arthropod), and earthworm (phylum:
Animal Body Plans
Animal Symmetry
Symmetry = balanced proportions / correspondence in size & shape of parts on opposite sides of a median plane
o Asymmetrical Bodies

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Not balanced
No plane
through which they can be divided into identical halves
o Spherical symmetry
plane passing through the center
divides the body into equivalent/mirrored halves
Best suited for floating or rolling organisms
Example: some protozoans; rare in animals
o Radial symmetry
Forms that can be divided into similar halves by
more than 2 planes passing through the longitudinal axis
Best suited for: sessile, free-floating, or weakly swimming organisms
In sessile forms, the aboral (region opposite the mouth) surface is used for attachment
Common in: Tubular-shaped, vase-shaped, or bowl-shaped animals which in one end of the longitudinal axis is
usually the oral surface (mouth)
Example: hydras & jelly fishes (phylum: Cnidaria), sea urchins (phylum: Echinodermata), and some sponges
(phylum: Porifera)
Radial symmetry: Biradial symmetry
Some part that’s single or paired can be divided into
1 or 2 planes passing through the longitudinal
axis to
produce mirrored halves
Example: sea walnuts and comb jellies (phylum: Ctenophora)
o They’re roughly globular in form but have a pair of tentacles
Example: echinoderms are primarily bilateral (during larval stages) and secondarily radial (during adult stage)
Radiata: animals that are primarily radially symmetrical
Example: phyla Cnidaria and Ctenophora
o Bilateral symmetry
Forms that can be
divided along sagittal plane into 2
mirrored portions
i.e. left & right halves
major innovation because allows organisms to be better fitted for directional movement
strongly related to cephalization
cephalization = differentiation of a head end
advantageous on animals moving in its environment head first
o grants ability to sense and respond to environment
accompanied by differentiation along the anteroposterior axis
o i.e. polarity
o important during development where the poles produce gradient of chemical signals
regions of body develop differently
Bilateria: animals that are bilaterally symmetrical
Development of Animal Body Plans
o Zygote
Formed post-fertilization of egg
Single large cell
o Blastomere
Formed post-cleavage of zygote
Cleavage = orderly sequence of cell division
o Adult structures do not form until well after cleavage
o Occurs in different ways:
Poriferans & Cnidarians lack distinct cleavage pattern
Bilateria exhibit either radial or spiral cleavage
o Radial Cleavage
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