I don't want to go into changing this without further ado, but are the following not all essentially the same:
- A hard outside covering, as of a fruit, nut, or animal.
- A pod.
- The hard covering of an egg.
- The hard calcareous or chitinous external covering of mollusks, crustaceans, and some other invertebrates. In some mollusks, as the cuttlefishes, it is internal, or concealed by the mantle. Also, the hard covering of some vertebrates, as the armadillo, the tortoise, and the like.
Is this the following not also the same? You either have an empty shell, in the same sense as all the above, or a live clam, mussel, oyster, etc.
- Hence, by extension, any mollusks having such a covering.
So all the above could without loss be subsumed in:
- A hard outside covering, as of some fruits, nuts, eggs, and terrestrial and marine animals.
Perhaps "pod" is different, but is this an example of a shell?