Fig. 5From: Homology problems in cephalopod morphology: deceptive (dis)similarities between different types of ‘caecum’The classical evolutionary scenario (scenario 1) implicating a direct derivation of the spirulid from the ammonoid caecum. Focus is made on the ventral shell part. ca caecum, co conotheca, cr1-3 connecting rings 1–3, pro primordial rostrum, pw protoconch wall, ps proseptum, psi prosiphon, s1-3 septa 1–3, sh sheathBack to article page