Complex sentence: They may be easily recognised by the fact that the third pereiopod (walking leg) is greatly enlarged; compare the lobsters and crabs in which it is the first pereiopod that is much bigger than the others.
Split into simpler sentences: They may be easily recognised by their third pereiopod (walking leg), which is greatly enlarged. In the lobsters and crabs, it is the first pereiopod that is much bigger than the others.