Sea Urchin Fertilization and Embryology Lab

I've been having some difficulties coming up with a few of the remaining answers to questions in my Sea Urchin lab:

1. What would happen if the sperm could not get into the egg? Alternatively, what would happen if the fertilization membrane did not form?

2. Adult echinoids are pentameric or pentaradial in their symmetry. What is the symmetry of the larvae? What does that mean for the process of metamorphosis from larvae (pluteus) to adult echinoid?

3. Sea urchin larvae are used in toxocological studies as a model for human development yet when you look at adult urchins and humans they are quite different. How may the use of these larvae be justified as human model organisms? What are advantages of using a sea urchin model in toxicological studies?

4. Why do sea urchins have external fertilization? What are the adaptive advantages of external fertilization? What are the adaptive advantages of internal fertilization?

5. What would happen after 24 hrs. if you were to dissect some of the two cell stages? Why?