Crustacea

Crustacea, the largest class of the phylum Articulata, including the Crabs, Lobsters, Shrimps, Barnacles, Water Fleas, Sand Hoppers, etc. They are mostly aquatic, and breathe by gills or the general surface of the body. They have two pairs of antennas or feelers, both of which obtain their nerves from the mass above the oesophagus known as the brain (a "syncerebrum"). The thorax (the middle division of the body) is more or less united to the head, and carries a series of jointed limbs; the abdomen is usually also segmented and limb-bearing. The external form varies greatly; segmentation is usually well marked, but in one or two groups, such as the Ostracods, it is never seen, and in others it is rudimentary. Most Crustacea are, however, composed of a series of more or less similar segments; they are typically cylindrical or depressed rings bearing at the lower margins a pair of jointed appendages. The number of segments varies greatly; in the higher orders, such as the Decapods, the number is fixed at twenty (five in the head, eight in the thorax, and seven in the abdomen) as in Crayfish (q.v.); in these the appendages are altered into feelers, jaws, claws, walking limbs, accessory sexual organs, and swimming structures. In other orders the number of segments may be much greater; thus, it is over sixty in Apus, a form common in fresh water in Northern Europe. Externally the Crustacea are nearly always protected by a hard outer crust, which is usually periodically thrown off (ecdysis). The appendages are always jointed and consist of a basal joint (or propodite) and two branches, the exo- and endopodites; in the first antenna and in the Ostracods there is typically only one branch. In Apus there is more than one pair of appendages on each of the hinder segments of the thorax. Respiration is mostly by branchiae or gills, but it is cutaneous or effected by the general surface of the body in the Copepods and Barnacles; it is also rectal in the Phyllopods, Cladocerae, Crayfish, etc., water being periodically admitted to and forced from the anus. The body cavity (q.v.) usually is of the type known as a "pseudocoele" or false coelome, but in the Prawns there is a large true coelome or "archicoele" (q.v.). The eyes are simple or compound; there is usually one pair but may be more, or there may be one unpaired median eye. In some of the deep sea Crustacea, such as the Euphasidae, there are also scattered structures which are either eyes or luminous organs at the bases of the gills and limbs. There are many cases of degeneration in the Crustacea; thus the subterranean forms have lost their eyes. Parasites, e.g. the Rhizocephala, have lost the mouth and digestive apparatus; the most striking case is that of the complemental males of the Cirripedia, small dwarf forms, which live only to fertilise the female. Development is very seldom direct, and there is usually a well-marked metamorphosis; most of the Entomostraca are hatched into the Nauplius (q.v.) form; most of the Decapods commence life as Zooea (q.v.), but a few, such as Penaeus and Euphasia, have a Nauplius stage. The Crustacea are divided into two sub-classes - the Entomostraca (q.v.) and the Malacostraca (q.v.); the former are all small simple forms, with a variable number of segments. This includes four orders - the Phyllopoda (q.v.), e.g. the Fairy Shrimp, the Water Fleas etc.; the Ostracoda (q.v.), including the Cyprids; the Copepoda (q.v.), e.g. Cyclops; and the Cirripedia (q.v.), e.y. the Acorn Shells and Barnacles. The Malacostraca have nineteen or twenty segments and are larger. This includes three groups - (1) the Leptostraca (q.v.), with the interesting genus Nebalia; (2) Arthrostraca (q.v.), including the Amphipoda (q.v.) (sand shrimps, etc.), and Isopoda (q.v.), e.g. the "Water Slaters"; and (3) Thoracostraca - this is divided into the Sessile-Eyed Crustacea (Edriophthalmata), including the Cumacea and Stomatopoda (q.v.), e.g. Squilla, and the Stalked-Eyed Crustacea (Podophthalmata), including the Schizopoda (q.v.) as Mysis, and the Decapoda (q.v.), e.g. Crabs, Lobsters, Prawns, etc. The Crustacea are world-wide in distribution, but are mainly marine; the Branchiopoda (q.v.) live exclusively in fresh water or salt lakes and marshes; some Amphipoda, Isopoda, and Decapoda are terrestrial. The Phyllopods commence in the Cambrian period, the Decapods in the Carboniferous, and the Cirripedia and Ostracoda in the Silurian. The Trilobites are now generally referred to the Arachnida.