Complex sentence: Their common name opossum shrimps stems from the presence of a brood pouch, or "marsupium", in females, and the fact that the larvae are not free swimming characterises this order.
Split into simpler sentences: Their common name opossum shrimps stems from the presence of a brood pouch or "marsupium" in females. The fact that the larvae are reared in this pouch and are not free swimming characterises this order.