Crustaceans:  

Crustacean, common name for members of a subphylum of mainly aquatic arthropod animals with jaws and two pairs of antennae, such as the crab, lobster, and shrimp. They are among the most successful animals, dominating the sea much as insects dominate the land. The majority of individual animals in the world are marine crustaceans which belong to the copepod subclass of the crustacean subphylum. Crustaceans are also successful in fresh water; a few, such as woodlice, are also abundant in moist land environments. Although most crustaceans are small, they have a wide range of body form and habits, and the class includes larger invertebrates such as lobsters up to 60 cm (24 in) long and a spider crab with a leg span of 3.6 m (12 ft). The subphylum contains about 26,000 known species.