A crustacean falls into the insect and spiders family. It is an insect with segmented body which has a very big difference from the original spider family. And there are nearly 25000 species of crustacean. And majority of crustacean live in water than on land. The ones who feed on wood lice reside on land. The crustaceanâs body is made of 50 different segments. But the higher crustacean have 19 different segments. Its body has three segments. One is the head, thorax and then the abdomen. The head and the thorax are a single unit called as the cephalothorax. It usually has a hard substance that covers the body. It appears as an external skeleton which protects it like an armor. But this skeleton does not grow with the body of the animals.