I should mention, there's a few arachnids out there that do lay eggs and the young still ride around after they hatch -- the best examples I can think of being the Amblypygids (tailless whipscorpions) and the Uropygids (vinegaroons). In those cases, the eggs are held in a jellylike sac from beneath the mother's abdomen, so you may not know that you have babies until they show up on her back.