Answer:
The major difference in fertilization between aquatic and terrestrial animals is the problem of getting the sperm to the egg. It has to swim through a liquid environment which is very easy in aquatic habitats but essentially impossible for terrestrial ones unless the sperm is deposited internally along with some liquid produced by either parent. An alternative is for the male to package the sperm in a casing which the female inserts into an internal cavity where appropriate liquids are secreted.
