Crustaceans are specialized for life in water and possess gills to obtain oxygen. Their chitinous skeleton is usually hard-ened by calcium carbonate. The ap-pendages are specialized for swimming,crawling, attaching to other animals,mating, and feeding. Crustaceans possesstwo pairs of antennae(Figs. 7.28 and7.32), which are usually involved in sens-ing the surroundings. There may be as many as 150,000species of crustaceans, mostly undiscov-ered. Most are marine. Whereas insectsare by far the dominant arthropods onland, crustaceans are their counterparts atsea. The crustacean body plan is repeatedin myriad forms, from the familiar to thenot so familiar.