The number of segments a crustacean has varies greatly depending on the species, but it is possible for all the segments of a crustacean to have appendages. The type of appendage differs according to whether it is on the region of the head, thorax or abdomen.
The three regions of a crustacean's body may altogether have 19 to 50 segments. The head segments may have two pairs of antennae, which are used as sensory organs, mandibles, which are mouth parts similar to jaws, and maxillae, which help transfer food to the mandibles. The thorax segments have various types of legs, which may consist of pereiopods, which are walking legs or may also bear claws and assist in gathering food, and maxillipeds, which assist in feeding. The abdominal segments include pleopods, which are usually swimming legs, and a telson, which holds the anus.
In some crustaceans, the head and thorax are fused together to form a single piece called the cephalothorax. This is covered by an external skeleton, or carapace, which does not grow along with the animal, but must be periodically shed and replaced. Appendages often change shape and function as a crustacean grows. For example, in some types of larvae, antennae might be used for swimming and feeding, while in mature animals they are used only as sensory organs.