 In contrast to their earth-bound relatives, tortoises, sea turtles do not have the ability to retract their heads into their shells.Their plastron, which is the bony plate making up the underside of a turtle or tortoise's shell, is comparably more reduced from other turtle species and is connected to the top part of the shell by ligaments without a hinge separating the pectoral and abdominal plates of the plastron.Sizes among the seven species of sea turtles range from 71–213 cm; for example, the smallest turtle species in the family Cheloniidae, the Kemp's Ridley, only has a shell size of about 75 cm and a weight of 50 kg.