UntitledCambrian to the Carboniferous eras. They have no direct descendantsto-day. They were jointed-footed animals, allied to Crustaceans andperhaps also to King-crabs. They were able to roll themselves up intheir ring-armour.][Illustration: _Photo: British Museum (Natural History)._THE GAMBIAN MUD-FISH, PROTOPTERUSIt can breathe oxygen dissolved in water by its gills; it can alsobreathe dry air by means of its swim-bladder, which has become a lung.It is a _double-breather_, showing evolution in process. For sevenmonths of the year, the dry season, it can remain inert in the mud,getting air through an open pipe to the surface. When water fills thepools it can use its gills again. Mud-nests or mud encasements with thelung-fish inside have often been brought to Britain and the fish whenliberated were quite lively.][Illustration: THE ARCHÆOPTERYX(_After William Leche of Stockholm._)A good restoration of the oldest known bird, Archæopteryx (JurassicEra). It was about the size of a crow; it had teeth on both jaws; it had
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