Deep beneath the Devonian ocean lurked the largest arthropod to have ever lived: the eurypterid. Sporting a javelin lancet serrated with venom teeth tipping a heavily armored tail, the enormous sea scorpion did not live in fear. The rise of the eurypterid heralded the first true great predator of our planet. The colossal beast reigned over the waters of the world long before the first foot fell upon land, enduring for great ages uncontested by any organism dominant and supreme. Surely in a world aeons from mammalian ancestors, overrun with fishes tenderly armored with leather bodies did no creature contest the capital horror present.
With mouths of spiders these marine dictators ruled omnipotent, boasting enlarged pedipalps enormous and sharp, cutting like a vortex of knives the primitive life yet blossoming beneath them. Claws the size of chainsaws greeted those unlucky enough to succumb to the mechanical separation awaiting them. An exoskeleton impervious to pain or scar covered the great lengths of this nimble leviathan. Great plates concealed with tight spaces the powerful muscles swimming swiftly with oarlike feet soaring. A maw the size of a basketball.
Adamantine nightmares of station wagon length half buried lined the steaming shores of that alien ocean forgotten so long ago, searching patiently with ambush tension the ambient apprehension so diffused through a godless sea. Our stirring notochords do not pray to those who watch over us. The behemoth weight of that hateful creation grows loathsome and impossible, for surely do not the eurypterid kings now enslave those human vassals beneath the surface of their seas, and we, supreme, now see a land far from that world, erected so by languages built and structures to fill the horizon, though why this oppression so subsided, and why crabs of colossal size so declined comes from the gills.
The arthropod anatomy proves pinnacle in precambrain technology. The copepod is one of the most wildly successful organisms in the history of the world even to this day. An armored carapace provides reasonable defense to all creatures of comparable size, while an immediate respiratory system spurs an active metabolism. Internalization of the organs provides a safe space for evolution to advance, and complex systems develop as a result. Fortunately, these ideal conditions proved the undoing of the arthropod. An outer shell is non-permeable, and in the race for supremacy, a non-permeable outer layer separates the individual from the masses. In isolating the organism, however, the myriad functions of the body become increasingly less simple. Gone are the days of assimilating meals epidermically, and respiration is no simple matter of diffusion. Little spiracles breathe slowly along the thorax of every arthropod, filling them with tubes of oxygen thin and branching.
This simple solution to respiration works well at first, but little holes riddling the bodies of creatures the size of elephants becomes absurd. The largest eurypterids were a single custard from suffocating beneath their own weights. Even in the early world of a dense atmosphere heavy and pressurized, the spiracle respiration system reaches a size limit, which is why insects and spiders today grow not much larger than a foot in length. Inasmuch, the sea scorpion was not only the largest arthropod to have ever lived, but it is the largest arthropod that ever will.