Complex sentence: It appears that most of Burrough's Martian creatures are roughly equivalent to those found on Earth, though most seem to have multiple legs (usually a total of six limbs, but sometimes as many as ten) and all are egg-laying.
Split into simpler sentences: It appears that most of Burrough's Martian creatures are roughly equivalent to those found on Earth. Though in general, most seem to have multiple legs and all are egg-laying.