Answer:
Skeletal
Explanation:
An invertebrate is "an animal lacking a backbone, such as an arthropod, mollusk, annelid, coelenterate, etc." The backbone is part the skeltal system, which povides structure. Invertebrates often have a non-skeletal outer shell to provide structure or live in an enviorment where they don't need it (i.e. the shell of a snail and the water enviorment of an octopus).
