Original: Septum ( marine biology ) : walls between each chamber , or siphuncle , in shells of nautiloids , ammonites , and belemnites ; i.e. cephalopods that retain an external shell .
Simplified: Cephalopod Septa : walls between each chamber , or siphuncle , in shells of nautiloids , ammonites , and belemnites ; i.e. cephalopods that retain an external shell .