Trichoplax adhaerens (Tp) is the only living species in the phylum Placozoa. 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. They move using cilia, and any "edge" can lead. Tp 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. Tp sperm cells have never been observed, nor have embryos past the 64-cell (blastula) stage.
If Tp sperm are observed by future researchers, how many chromosomes should be found in a Tp sperm nucleus?
A) 2B) 3
C) 6
D) 12
Answer: C
In how many of the following ways is Tp unlike the typical animal?
1. Tp is multicellular.2. Tp lacks muscle and nerve cells.
3. Tp has cilia.
4. Tp has a different place where digestion of food occurs.
5. Tp lacks cell walls.
A) only one way
B) two ways
C) three ways
D) four ways
E) all five ways
Answer: B
On the basis of information in the previous paragraph, which of these should be able to be observed in Tp?
A) the act of fertilizationB) the process of gastrulation
C) eggs
D) All three of the responses above are correct.
E) Two of the responses above are correct.
Answer: C
In its native environment, a Tp cell neither gains nor loses water. What should one expect to occur when Tp is placed into fresh water?
A) no change from the above, as fresh water is its native environmentB) lysis
C) plasmolysis
D) slight shrinkage
Answer: B
Tp's body symmetry seems to be most like that of A) most sponges.
B) cnidarians.C) worms.
D) tetrapods.
E) Two of the responses above are correct.
Answer: A
In an experiment, several Tp individuals were stained different colors. The stained individuals were then passed through a strainer, disaggregated to the level of single cells, and collected into a common container of seawater. Which subsequent finding would be most surprising if the Tp individuals used in this experiment had been produced by sexual, rather than asexual, means?
A) If all of the cells from a given individual reaggregated to form the same individual, and if each cell had retained its original identity, as far as cell type goes.
B) If all of the cells from a given individual reaggregated to form the same individual, but if each cell had a different identity than it had before disaggregation.C) If cells from different original individuals reaggregated together to form new individual organisms.
D) If cells from different original individuals reaggregated together to form new species.
Answer: C
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