Decapoda

Decapoda, the name of two orders of Invertebrates - one of Crustacea and the other of Cephalopoda. (1) The former is the better known of the two sub-orders of the Podophthalmata or "Stalked-eyed Crustacea," and it includes the crabs, lobsters, prawns, and many of the most familiar larger English Crustacea. The characters of the order are the fusion of the thoracic somites into a distinct head and the fact that the five last pairs of feet on the thorax are seven-jointed and of but one branch (i.e. are uniramose). It is from these five pairs of similar and approximately equal legs that the class derives its name. The branchiae or gills are borne on the legs or the sides of the body, and are enclosed in a special cavity. The head and thorax are protected by a strong shield known as the cephalothorax; in the Brachyura the abdomen is turned up under the body, and is therefore also protected by the carapace. The Decapoda differ so much among themselves, and the order includes so many important crustaceans, that further details are given under the sub-headings: - There are two sub-orders, the Macrura or "long-tailed" group and the Brachyura or "short-tailed." The former includes the" lobster, crayfish, shrimps, prawns, hermit crabs, tree crabs (Birgus), etc. The Brachyura includes the true crabs, the sponge crabs (Dromia), the land crabs (Gecarcinus), etc. To each of these reference should be made. (2) Cephalopoda Decapoda, a sub-order of the Dibranchiata or two-gilled Cephalopods. In this group the animals have eight equal arms and two longer arms or tentacles with expanded ends; the function of the latter pair is reproductive. Correlated with the possession of these tentacles are the presence of fins and an internal skeleton and the fact that the suckers on the arms are stalked or "pedunculate." The Decapoda are divided into four families: - The Belemnitidae or Belemnites (q.v.), which are all extinct; the Spirulidae, containing Spirula (q.v.), a small form common in Southern seas, which is provided with a delicate spiral internal shell; the OEgopsidae, a primitive family, with an eye of which the cornea is perforate; Ommastrephes is the best known genus. The last family is the Myopsidae, in which the eye is closed; it contains the sub-families Sepiadae, with the Cuttle-fish (q.v.), and the Teuthidae, with the Calamaries (q.v.) or Squid as its best known representative.