The above-named fossil groups are included by zoologists in the sub-kingdom Arthropoda ("joint-footed animals"). The Arthropods possess a body and limbs composed of a number of jointed segments covered externally with a hard, shelly material and separated by a softer, flexible skin. They have no internal skeleton, and therefore the only portion which can be preserved in the fossil state is the harder part of the outer covering. Under exceptional conditions of fossilisation, however, even frail insects such as ants, wasps and dragon-flies are sometimes found more or less wholly preserved and showing their original minute structure. =Subdivisions of Arthropoda.--= The principal representatives of the group of the Arthropods which are found as fossils include the Trilobites; various Crustacea proper, as Crabs, Lobsters, Shrimps, Pod-shrimps and Water-fleas; the Insects; and occasionally Spiders and Scorpions (Arachnida). The King-crabs and Eurypterids (as the extinct _Pterygotus_) form a separate sub-class, the Merostomata, which are placed by some authors in the group of Spiders and Scorpions: their remains date back to the time when the older Palaeozoic strata were deposited. =Crustacea, an Archaic Group.--= A typical division of the Arthropod group, and one which was well represented from the earliest period up to the present day, is the _CRUSTACEA_. As the name denotes, these animals are generally invested with a strong shelly covering or "crust," usually of horny or chitinous material, which in some forms is strengthened by deposits of phosphate of lime. Of the horny condition of the shell the groups of the bivalved Crustacea (Ostracoda) and the "water-fleas" (Entomostraca) supply notable instances; whilst the limy-structured shell is seen in the common crab. Some authorities separate the great extinct group of the Trilobites from the rest of the Crustacea; but it will here be convenient, in a preliminary study, to consider them together. =Development of Crustacea.--= The development of the lower forms of the Crustacea is interesting, from the fact that the young usually escapes from the egg in a larval state known as a "nauplius." In this stage there are no segments to the body, and but a solitary median eye, such as may be seen in the common water-flea known to microscopists as _Cyclops_. The three pairs of appendages seen in this larval crustacean represent the two pairs of antennae and the jaws or mandibles of the full-grown form. Among the higher Crustacea, however, there is no larval form; the young escaping from the egg in a more or less highly developed condition resembling the adult. The group of the Crabs, Lobsters and Shrimps (or Decapoda, _i.e._, having ten ambulatory feet) exhibit a larval stage in which the young form ("zoea") has a segmented abdomen and seven pairs of appendages. =Trilobites.--=