Anostraca is one of the four orders of crustaceans in the class Branchiopoda; its members are referred to as fairy shrimp. They live in vernal pools and hypersaline lakes across the world, and they have even been found in deserts, ice-covered mountain lakes and Antarctic ice. They are usually long (exceptionally up to ). Most species have 20 body segments, bearing 11 pairs of leaf-like phyllopodia (swimming legs), and the body lacks a carapace. They swim "upside-down" and feed by filtering organic particles from the water or by scraping algae from surfaces. They are an important food for many birds and fish, and some are cultured and harvested for use as fish food. There are 300 species spread across 8 families. Description The body of a fairy shrimp is elongated and divided into segments. The whole animal is typically long, but one species, Branchinecta gigas does not reach sexual maturity until it reaches long, and can grow to long. The exoskeleton is thin and flexible, and lacks any sign of a carapace. The body can be divided into three distinct parts (tagmata) – head, thorax and abdomen. Head The head is morphologically distinct from the thorax. It bears two compound eyes on prominent stalks, and two pairs of antennae. The first pair of antennae are small, usually unsegmented, and uniramous. The second pair are long and cylindrical in females, but in males they are enlarged and specialised for holding the female during mating. In some groups, males have an additional frontal appendage. Thorax and abdomen The thorax of most anostracans has 13 segments (19 in Polyartemiella and 21 in Polyartemia). All but the last two are very similar, with a pair of biramous phyllopods (flattened, leaf-like appendages). The last two segments are fused together, and their appendages are specialised for reproduction. Most anostracans have separate sexes (gonochorism), but a few reproduce by parthenogenesis. The abdomen comprises 6 segments without appendages, and a telson, which bears two flattened caudal rami or "cercopods". Internal anatomy The head contains two digestive glands and the small lobate stomach into which they empty. This is connected to a long intestine, which terminates in a short rectum, with the anus located on the telson. The haemocoel of anostracans is pumped by a long, tubular heart, which runs through most of the animal's length. A series of slits allow haemocoel into the heart, which is then pumped