SUB-KINGDOM ARTHROPODA CLASSIFICATION Class =CRUSTACEA=. Sub-class =ENTOMOSTRACA=. Order =Astracoda=--Free. Body enclosed in a bivalve shell. Order =Copepoda=--Free. Five pairs of feet adapted for swimming. Order =Cirripedia=--Sessile. Enclosed in a shell of many valves. Order =Branchiopoda=--Free. Gills attached to feet. Sub-class =MALACOSTRACA=. Division =EDRIOPHTHALMATA=, or Sessile-eyed Crustaceans. Order =Isopoda=--Body flattened. Seven pairs of legs--equal. Order =Amphipoda=--Body flattened laterally. Legs adapted for both walking and swimming. Division =PODOPHTHALMATA=, or Stalk-eyed Crustaceans. Order =Stomapoda=--Anterior appendages directed towards the mouth. Order =Schizopoda=--Cleft-footed Crustaceans. Order =Decapoda=--Ten-footed Crustaceans. Sub-order =Macrura=--Great-tailed. Lobsters, &c. Sub-order =Brachyura=--Short-tailed. Crabs. Class =ARACHNOIDEA=. Order =Scorpionidæ=--Scorpions. Order =Araneidæ=--Spiders. Order =Acarina=--Mites. Class =MYRIOPODA=. Order =Chilopoda=--Centipedes. Order =Chilognatha=--Millepedes. Class =INSECTA=. Order =Rhynchota=--Imperfect metamorphoses, suctorial mouth. Bugs. Order =Thysanura=--Imperfect metamorphoses. No wings. Divided tail. Spring-tails. Order =Euplexoptera=--Abdomen with terminal forceps. Earwigs. Order =Thysanoptera=--Four equal membranous wings. Thrips. Order =Orthoptera=--Anterior wings usually shorter and firmer. Grasshoppers, &c. Order =Neuroptera=--Two pairs of glassy wings--equal. Order =Trichoptera=--Wings unequal, clad with hairs or scales. Caddis flies. Order =Aphaniptera=--No wings, no compound eyes. Fleas. Order =Diptera=--Two membranous wings. Flies. Order =Lepidoptera=--Wings clad with scales. Butterflies and Moths. Order =Coleoptera=--Fore wings hard and horny. Beetles. Order =Hymenoptera=--Four membranous wings. Larvæ, footless grubs. Ants, Bees, &c. CHAPTER XIV _MARINE VERTEBRATES_ The vertebrates form the highest sub-kingdom of animal life--the sub-kingdom to which we ourselves belong, the chief distinguishing characteristic of the group being the presence of an internal skeleton, the principal part of which consists of a rod or column of cartilaginous or bony material running along the dorsal side of the body, known as the _vertebral column_.