The Yerushalmi (p. 8a, ch.1 halacha 3) says that the hillazon has bones. The Murex doesn’t have bones, so how do you explain this?
The Yerushalmi says that anything without “gidim ve’atzamot” (sinews and bones) cannot live more than 6 months. Now since we know that the Murex trunculus lives for over 7 years this must mean that it has “gidim ve’atzamot” – so presumably the shell which is like it’s skeleton was considered by the Yerushalmi to be its “bones” (see See Rabbi Menachem Burshtein’s “Hatekhelet”, p. 35, and especially R Herzog’s explanation of the issue on p. 389 of the same book). – Mois Navon