sentence2_structure: Vertebrata<>are characterized<>by the presence of backbone
sentence1: Plasmalogens are glycereophospholipids characterized by the presence of vinyl ether bond at the sn-1 position of glycerol backbone and an ester bond at the sn-2 position.
sentence2: Vertebrata are characterized by the presence of backbone.
question: Vertebrata are characterized by the presence of what?