Final answer:
In arboreal mammals, a terminal appendage often refers to the prehensile tail used for climbing and balance. Whereas in aquatic mammals like whales, hind leg bones are vestigial structures. These appendages reflect different ecological adaptations and evolutionary histories in mammals.
Step-by-step explanation:
A terminal appendage found in certain mammals could refer to various structures depending on their function and location. In the context of arboreal mammals, such as those from the genus Gorilla, this appendage is often the prehensile tail. This tail is specially adapted for grasping or holding objects and is particularly useful for climbing and balancing within their tree-dominant habitats. These adaptations enable the primates' unique method of locomotion called brachiation, or arm swinging, an energy-efficient way of moving through the forest canopy using their very long arms.
Meanwhile, in aquatic mammals like whales, evolutionary processes have led to the modification of certain appendages. For example, hind leg bones in whales are vestigial structures, remnants of a terrestrial past that are no longer needed for their current aquatic lifestyle.
In eutherian mammals, which form a large subgroup including primates and cetaceans, evolutionary adaptations of limbs and tails reflect the diverse ecological niches they occupy. Such structural variations can be seen as evidence of a shared common ancestor among different species of mammals, pointing to the deep evolutionary roots of these appendages. Be it for climbing, flying, swimming, or even as vestigial remnants, these appendages offer insight into the history of mammalian evolution.