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Chapter 3

BIOL 2030 Chapter Notes - Chapter 3: Sagittal Plane, Blastocoel, Gastrointestinal Tract

Course Code
BIOL 2030
German Reyes

of 3
There are 5 major levels of organization in organismal complexity:
1. Protoplasmic level: unicellular organisms.
2. Cellular level: colonial (aggregation of unspecialized cells, all able to reproduce) or multicellular
(aggregation of cells that are functionally differentiated, reproduction is limited). May secerete
an extracellular matrix (ECM).
3. Cell-tissue level: similar cells aggregate into definite patterns or layer, thus becoming a tissue.
Eg. jellyfish
4. Tissue-organ level: organs usually contain more than one kind of tissue and have a more
specialized function than tissue. Eg. Flatworms
5. Organ-system level: organs work together to perform a function. Systems are associated with
basic body functions: respiration, digestion. Eg. Frogs
Asymmetrical bodies are not balanced, there is no plane through which they can be divided into
identical halves.
Spherical symmetry means that any plane passing through the center divides the body into equivalent
halves. (more in protozoan groups, suited for floating or rolling)
Radial symmetry: can be divided into similar halves by more than two planes passing through the
longitudinal axis. Eg. Jellyfish. Usually one end of the longitudinal axis is the mouth. Biradial symmetry:
one or two planes passing through the longitudinal axis produce mirrored halves.
- Radial and biradial animals are usually sessile, free-floating or weakly swimming.
Bilateral symmetry: can be divided along a sagittal plane into two mirrored portions, right and left
halves. Anterior is the head end, posterior is the tail end, dorsal is the backside, and ventral is the front
or belly side. Medial is the midline of the body, lateral is the sides.
- Bilateral symmetry animals are much better fitted for directional (forward) movement than
radial symmetry animals.
- Cephalization: differentiation of a head end. Associated with bilateral sym.
Two types of cleavage: radial and spiral. In radial cleavage, it produces layers of cells on top of each
other in an early embryo. It occurs with regulative development; each blastomere of the early embryo
can adjust its development into a complete and well-proportioned embryo. Typically seen in chordates
and hemichordates. In spiral cleavage, the eight-cell stage cleavage produces two quartets of cells that
come to lie not on top of each other but in furrows between cells. The eggs tend to pack their cells
tightly together. Most have mosaic development; the organ-forming determinants in the egg cytoplasm
become strictly localized in the egg. Typically seen in molloscs. Cells move clockwise.
- Gastrula: embryo has two germ cells layers: ectoderm and endoderm. The ectoderm surrounds
the blastocoel (the cavity in blastula/embryo). Endoderm syrrounds an inner body cavity:
gastrocoel. Gastrocoel becomes the gut cavity in most animals.
- Blastopore is the opening of a gut
- A gut that only has one opening is a blind gut
- Pseudocoelom: mesoderm lines the outer edge of the blastocoel.
- Acoelomate: the mesoderm completely fills the blastocoel. The only body cavity in this case is
the gut tube.
Coelomate: the blastocoel fills with mesoderm and then a new cavity forms inside the mesoderm. This
differs from pseudocoelom, which has mesoderm on only the outer edge of the cavity. Coelomate can
form in two ways: schizocoely; the coelom arises from splitting of mesodermal bands that originate
when cells in the blastopore region migrate into the blastocoel. Another way is enterocoely; the coelom
comes from pouches of the archenteron, or primitive gut, which push outward into the blastocoel.
- Fluid-filled coelom serves as a hydrostatic skeleton.
Metamerism: segmentation; serial repetition of similar body segments along the longitudinal axis of the
body. Each segment is called metamere/somite.
- In Bilateria there are two groups of animals: protostomia and deuterostomia.
Deuterostome body plans: the blastopore becomes the anus. The mouth forms secondarily. They have
radial cleavage. Coelom forms by outpocketing/enterocoelous. Seen in phylum Echinodermata,
Hermichordata, Chordata, and Xenoturbellida.
Protostome body plans: have spiral cleavage. The blastopore becomes the mouth. The anus forms
secondarily. When coelom is present it forms by splitting/schizocoelous. Seen in Nematoda,
- There are subgroups for protostomes: one is ecdusozoa; the animals molt their cuticles
(anthropods, nematodes). Another is lophotochozoa; they have a horse-show shaped whorl of
tenticles called lophophore seen in lampshells. They also have a band of cilia on a larcal form
called a trochophore seen in snails and woms.
- A tissue is a group of cells specialized for performing a common function. Study of it is called
histology. There are four types of tissue: muscular, nervous, epithelial, and connective.
Epithelium is a sheet of cells that cover an external or internal surface. They are supported by collagen;
an extracellular matrix that is flexible. Depend on diffusion of oxygen and nutrients from underlying
tissues because no blood vessels go through it. Transitional epithelium: great stretching (in bladder).
Stratified squamous epithelium: go through continuous mitotic divisions and shed and replace cells.
Connective tissue: composed of extracellular fibers, ground substance (matrix) and few cells. Two kinds
of connective tissue are found in vertebrates: loose connective tissue; composed of fivers and both fixed
and wandering cells suspended in a syrupy ground substance (fat tissue). It anchors blood vessels,
organs, and nerves. Another is dense connective tissue; largely densely packed fibers. A lot of
connective tissue is made of collagen (protein). Other types of connective tissue include blood, lymph,
and interstitial fluid. Cartilage is a semirigid form of connective tissue with closely packed fivers
embedded in a gel-like ground substance. Bone is a calcified connective tissue containing calcium
around collagen fibers.
Muscle is the most common tissue in animals. There are three types: skeletal, cardiac and smooth
muscle. Skeletal and cardiac are striated muscles (have bands). Cardiac muscle is found only in the heart
of vertebrates, they are shorter than skeletal, and have only one nuclei per cell. They are called
involuntary muscle, skeletal are voluntary because it contracts when stimulated by nerves under
conscious cerebral control. Smooth muscle is the most common and is involuntary.
Nervous tissue receives stimuli and conducts impulses from one region to another. Neuroglia is a variety
of nonnervous cells that insulate neuron membranes and serve various supportive functions.
- A large animal will have less surface area relative to its volume than will a small animal of the
same shape.