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Remember that Latin doesn't just add an "s" to the end of a word to make it plural. Without going into the grammar rules, remember these tricky terms:
Watch out for the difference between species names and species identifiers. A species identifier is just one word (the second in a binomial name) while a species name is both parts of a binomial name. Let's work with an example. Take Elephas maximus. (That's an Asian elephant.) The name of its genus is Elephas; the name of its species is…Elephas maximus; and the name of its species identifier is maximus. The species name contains the genus name within it. The species identifier is just one word that doesn't mean much on its own because it can be used over and over for different species. For example, a Pecten maximus is a type of clam that obviously has very little in common with an elephant.
1. Remember that homology means true similarities based on common ancestry whereas homoplasy means "fake" similarities based on convergent evolution and reversals. Think of "plastic" things as being fake.
2. Shared characters are relative terms. Keep in mind that many of these phylogenetic terms are relative; that is, they depend on context. For example, if you are comparing fish to cats and dogs, then hair is a derived character shared by cats and dogs. In other words, cats and dogs share a common ancestor that had hair; fish had no such common ancestor. However, if you are comparing cats to dogs, hair is now considered a shared ancestral character because it is a trait that their shared common ancestor had.
Acoelomates are NOT animals with "a coelom." "A-" is being used here as a prefix that means "without." Another biology word that uses that prefix is "asexual," meaning reproduction that doesn't involve two genders.
All of those -trophs. It's hard to keep them straight. First of all, remember that "troph" means nourishment or food in Greek. Next, remember that auto- and hetero- pertain to carbon sources and chemo-and photo- pertain to energy sources. Autotrophs are autonomous/independent of other organisms; they do not rely on others to create organic compounds but can do it on their own. Heterotrophs get their carbon by digesting organisms (either autotrophs or their fellow heterotrophs) that already contain organic compounds. Chemotrophs are like batteries; they get their energy from chemicals. Phototrophs are like solar panels; they get their energy from the sun.
The Domain Archaea used to be called Archaeabacteria because scientists thought of the archaea as bacteria-like organisms. We now know that they are quite different and should not be called bacteria.
Remember that multiple taxonomic categories can be grouped together into categories, called clades, that are not part of the taxonomic hierarchy (they are not a domain, kingdom, phylum, class, order, family, genus or species). These categories are still important because they tell us a lot about the evolutionary history of the organisms within them. Protostomes and deuterostomes are an example of clades.