ANIMALS OF THE CAMBRIAN PERIOD e.g. Sponges, Jellyfish, Starfish, Sea-lilies, Water-fleas, and Trilobites] [Illustration: _Photo: J. J. Ward, F.E.S._ A TRILOBITE Trilobites were ancient seashore animals, abundant from the Upper Cambrian to the Carboniferous eras. They have no direct descendants to-day. They were jointed-footed animals, allied to Crustaceans and perhaps also to King-crabs. They were able to roll themselves up in their ring-armour.] [Illustration: _Photo: British Museum (Natural History)._ THE GAMBIAN MUD-FISH, PROTOPTERUS It can breathe oxygen dissolved in water by its gills; it can also breathe dry air by means of its swim-bladder, which has become a lung. It is a _double-breather_, showing evolution in process. For seven months 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 the pools it can use its gills again. Mud-nests or mud encasements with the lung-fish inside have often been brought to Britain and the fish when liberated were quite lively.] [Illustration: THE ARCHAEOPTERYX (_After William Leche of Stockholm._) A good restoration of the oldest known bird, Archaeopteryx (Jurassic Era). It was about the size of a crow; it had teeth on both jaws; it had claws on the thumb and two fingers; and it had a long lizard-like tail. But it had feathers, proving itself a true bird.] [Illustration: WING OF A BIRD, SHOWING THE ARRANGEMENT OF THE FEATHERS The longest feathers or primaries (PR) are borne by the two fingers (2 and 3), and their palm-bones (CMC); the second longest or secondaries are borne by the ulna bone (U) of the fore-arm; there is a separate tuft (AS) on the thumb (TH).] The rarity of direct traces of life in the oldest rocks is partly due to the fact that the primitive animals would be of delicate build, but it must also be remembered that the ancient rocks have been profoundly and repeatedly changed by pressure and heat, so that the traces which did exist would be very liable to obliteration. And if it be asked what right we have to suppose the presence of living creatures in the absence or extreme rarity of fossils, we must point to great accumulations of limestone which indicate the existence of calcareous algae, and to deposits of iron which probably indicate the activity of iron-forming Bacteria. Ancient beds of graphite similarly suggest that green plants flourished in these ancient days. Sec. 3 The Era of Ancient Life (Palaeozoic)