Exercise 5.4. Symmetry (Sample Set 4)
The growth from the single cell of a fertilized egg into a full-sized organism with multiple types of cells, and groups of cells that make structures (like a whole arm), is controlled in part by Hox genes. These are the architectural genes within DNA that determine the big picture structure of an organism. Different Hox genes have different strategies for assembling the organism, which can in turn control the symmetry of the final organism. For example, humans are bilaterally symmetrical in our outward appearance. The Hox genes which control our external development result in our two halves being almost mirror images.
Symmetry can be:
Bilateral: The left and right sides of an organism, or the top and bottom, or the front and back are mirror images of each other.
Radial: The organism is symmetrical in multiple wedges, radiating from a central point like the spokes of a wheel.
Pentameral: A type of radial symmetry. The organism is arranged in symmetrical “wedges” to make up a five-rayed appearance. E.g. starfish
Asymmetrical: No symmetry present.
What To Do
Inspect the fossil specimens in Sample Set 4 (images available below), paying special attention to the features you are asked to examine. Answer questions (a) to (f) as you work through Sample Set 4.
Note: If the image slide-decks fail to load, click the sample title to open the slide deck in a new window.
Sample Set 4 & Questions
A. Sample 10: Trilobite model
Sample 10 can be picked up but please handle with care.
Questions a & b
a. What kind of symmetry does this cast of a trilobite display?
b. Where does the plane of symmetry run? (I.e., if you had to draw a line such that the trilobite was symmetrical on either side, where would you draw the line?)
B. Sample 121-24: Trilobite
Sample 121-24 can be picked up but please handle with care.
Compare this real sample of a trilobite to the ideal model above. Parts of the trilobite exoskeleton have been distorted from the original animal, but the symmetry is still evident. In other words, This particular fossil is not perfectly symmetrical, but the animal it preserves was symmetrical. This is what you may find in many natural samples.
C. Sample 9: Shell
Sample 9 can be picked up but please handle with care.
Question c
D. Sample 30: Clam Shells
Sample 30 can be picked up but please handle with care.
Question d
The last two samples both look like what we might call a “clam”, and in fact they both would have lived in a similar manner to modern clams. However, we’ll see in Lab 6 that the difference in symmetry puts them in very different biological classifications.
E. Sample FDA10 & 10
These samples can be picked up but please handle with care.
Notice that in cross-section the structure of the organism radiates outwards from a central point.
Question e
F. Sample 8: Blastoid
These samples can be picked up but please handle with care.
Question f