Complex sentence: Belemnites (or belemnoids) are an extinct group of marine cephalopod, very similar in many ways to the modern squid and closely related to the modern cuttlefish, and like them the belemnites possessed an ink sac and ten arms (or tentacles).
Split into simpler sentences: Belemnites (or belemnoids) are an extinct group of marine cephalopod, very similar in many ways to the modern squid and closely related to the modern cuttlefish. Like them, the belemnites possessed an ink sac and ten arms (or tentacles).