Shell rudiment. Small internal structure made of calcium carbonate secreted by the mantle; some species do not have shells. Crop. Large sac located where food is held before being digested in the stomach.

What is a Hectocotylus used for?

A hectocotylus (plural: hectocotyli) is one of the arms of male cephalopods that is specialized to store and transfer spermatophores to the female.

Where is the shell in an octopus?

Its shell is inside of its body. Its shell has chambers and the shell has even evolved to protect its brain. That protection is an advantage. It's also an advantage for the octopus to have the shell inside of the body.