Manatees lack hind limbs as adults but have them as embryos, showing their clear connection to other tetrapods. These limbs are not needed for swimming. Manatees would share the other tetrapod characters shown in Fig. 26.12b since these would still be useful for life in the water. Early members of the manatee lineage would have had hind limbs, though probably reduced or otherwise modified ones which were lost over time as manatees evolved.
Work Step by Step
The key here is to see that embryos (review the textbook) can tell us more about relationships in some cases than adults since the embryos may retain features which are lost in the adults. Further, review the characters in Fig. 26.12b and ask yourself if having four limbs might still be useful in the aquatic life of a manatee.