Can a calm have hard external skeleton?
I assume calm means clam? In which case, a clam does have a hard external (skeleton?) shell of calcium carbonate. The shell is known as an exoskeleton.
Skeletons: All animals, including humans have skeletons. Humans have an internal skeleton. This means that our bones is on the inside of us. When an animal has an external skeleton it means that their skeleton is on the outside. These animals who have an external skeleton have what we commonly call shells. These shells are their skeleton which keeps their insides together. If you think of one of those big crabs and how there is…
They're invertebrates because they don't have an internal skeleton - like humans and mammals do. Instead, they have an external covering - called an exo-skeleton. By definition an invertebrate is an organism with no vertebra (backbone, spine). Arthropods don't have them nor need them as they use an external skeleton (exoskeleton) made from a hard protein (chitin), to provide the same functions as internal skeletons (endoskeletons) on vertebrates.