CHAPTER 3 NOTES
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.
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
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