The first group of arthropods here described is that of the _TRILOBITES_. These were so named on account of the three-lobed form of the body. This particular feature distinguishes them from the Crustacea proper; which includes the Phyllopods (with leaf-like limbs), as the freshwater _Estheria_, the Ostracoda or Bivalved Water-fleas, the Barnacles or Cirripedia and the Higher Crustacea (Malacostraca), including Shrimps, Crabs, and Lobsters, of which the oldest representatives are the Pod-shrimps (Phyllocarida). =Habits of Trilobites.--= The remains of these primitive but often strikingly ornamented crustacean-like animals, the trilobites, are found in comparative abundance in the limestones, mudstones, and even the sandstones of the older sedimentary rocks of Australasia. They were amongst the most prolific types of animal life existing in the seas of Palaeozoic times, and are especially characteristic of Cambrian, Ordovician and Silurian rocks. Trilobites, as a group, seem to have adapted themselves to almost all conditions of marine life: some are found in the hardened black mud of shallow waters, whilst others are to be looked for in the limestones and excessively fine sediments of deeper waters. In all probability certain of these forms crawled over the soft, oozy sea-bed in order to obtain their food, and consequently their remains in the stratified rocks would be restricted to the fine black shales; whilst the freely swimming forms could change their habitat at will, and would be found alike in sandy or clayey deposits. As some indication of their varied habits, the eyes of trilobites differ greatly in size. They are always compound like the eye of the house-fly, though of a semi-lunar shape. In some forms the eyes are very small or even absent, whilst in others they are exceedingly large and prominent. This latter feature probably indicates their frequenting moderately deep water. [Illustration: =Fig. 107--Diagram-restoration of an Australian Trilobite.= (Dalmanites meridianus, Eth. fil. and Mitch, sp.) To show the sutures or joints, and the structure of the back of the carapace. About 2/3 natural size. ] =Structure of Trilobites.--=