5. Which animals have true tissues, and which have organs? 6. What is the difference between radial and bilateral symmetry?
7. Distinguish between a coelom and a pseudocoelom.
8. What are the two main types of digestive tract?
9. What advantages does segmentation confer?

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the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.