Chapter 32 Questions
- One hypothesis suggests that the Cambrian explosion was caused by the rise of predator-prey relationships. What is the evidence that supports this hypothesis? What were the characteristics of the fossils found that would make biologist come up with this hypothesis? (5 points)
- a . The earliest macroscopic fossils of animals date back to ~560 million years ago. If fossils of bilaterally symmetrical animals were found to date back to 660 million years ago, would that be a surprise? Why/why not? (2 points)
b. If fossils of bilaterally symmetrical animals that must have lived on land were found to date back to 660 million years ago, would that be a surprise? Why/why not? (2 points)
- You have discovered a new species of eukaryote! However, you are unsure whether you have an organism that is a choanoflagellate or an animal. What would you do to determine whether this new discovery you have made is truly an animal? What characteristics would you be looking for? (5 points)
- Trichoplax adhaerens is the only living species in the phylum Placozoa in the animal kingdom. Individuals are about 1 mm wide and only 27 µm high, are irregularly shaped, and consist of a total of about 2,000 cells, which are diploid (2n = 12). There are four types of cells, none of which are nerve or muscle cells, and none of which have cell walls. Individual animals move using cilia, and any “edge” can lead. T. adhaerens feeds on marine microbes, mostly unicellular green algae, by crawling atop the algae and trapping it between its ventral surface and the substrate. Enzymes are then secreted onto the algae, and the resulting nutrients are absorbed. T. adhaerens sperm cells have never been observed, nor have embryos past the 64-cell (blastula) stage (in other words, there is no information on their reproduction or their embryonic development). (7 points)
- Using the information above and some research that you do on your own, what characteristics does this species have that make it an animal?
- Is it a eumetazoan based on your information? Why/why not? Explain your reasoning.
- What characteristics are unique to this species?
- You come across a bunch of zygotes in a lab that studies both starfish (in the phylum Echinodermata) and clams (in the phylum Mollusca). How could you use these zygotes as they develop into early embryos to determine whether they are starfish (sea stars) or clams? At what points in their embryonic development would you be able to tell which ones they were? (4 points)
- According to your textbook, the phylum Platyhelminthes belongs in the Lophotrochozoa. However, many biologists believe that animals in the Platyhelminthes are actually basal bilaterians (in other words, they diverged before the common ancestors to the Lophotrochozoans and Ecdysozoans did). Using information from both lecture and lab (and from outside sources), explain why some biologists believe that Platyhelminthes belongs in the Lophotrochozoa, while others believe they are basal bilaterians. (6 points).
- Suppose a researcher for a pest-control company developed a chemical that inhibited the development of an embryonic mosquito’s endodermal cells, such that the endoderm did produce cells, but the cells produced abnormal organs that did not function properly. Explain how do think each of the following would be affected and whether the chemical would work on the mosquitos due to their disfunction (4 points):
- Digestive tract
- Brain function?
