they migrate towards the coast. Then on three successive nights, when the moon is full and the tides are high, hundreds of thousands emerge from the sea. The females, their huge shells glinting in the moonlight, move towards the shore, dragging smaller males behind them. Sometimes four or five males, in their anxiety to reach a female, cling to one another and form a chain. As she reaches the edge of the water, the female half buries herself in the sand. There she sheds her eggs and the males release sperm. For kilometre after kilometre along the dark beaches, the living tide of horseshoe crabs is so thick that they form a continuous strip, like a causeway of giant cobbles. The breakers sometimes overturn them and they lie in the sand, with their legs waving, their stiff tails slowly swivelling, in an effort to lever themselves right side up. Many fail and are abandoned by the receding tide to die as thousands more swim in the shallows, pressing forward to take their turn.
This scene must have been enacted every spring for several hundred million years. When it began, the land was without life of any kind, and on such beaches the eggs were safe from sea-dwelling marauders. Perhaps this is why the horseshoe crabs developed the habit. Today beaches are not quite so safe, for hordes of gulls and small wading birds congregate to share the prodigious feast. But many of the fertilised eggs remain buried deep among the sand grains where they will stay for a month until, once more, high water reaches this part of the beach, stirring the sand and releasing the larvae to swim freely in the sea.
Although the trilobites were so successful, they were by no means the only armoured creatures to develop from the segmented worms. So did a group that must have been among the most alarming of all marine monsters – the sea scorpions, called scientifically the Eurypterids. Some grew to a length of two metres and were the largest arthropods ever known to have existed. However, in spite of their appearance and huge claws, many of them were filter feeders. Presumably, their fearsome claws were used in fights between one another rather than in subduing prey. Like the trilobites, they disappeared at the end of the Permian period.
One group related to the trilobites did however survive and today is extremely successful. They differed in one seemingly trivial but nonetheless diagnostic characteristic. They have not one but two pairs of antennae on their heads. They lived alongside the trilobites, comparatively unobtrusively for hundreds of millions of years, and then, when the trilobite dynasty came to an end, it was they who took over. They are the crustaceans. Today there are about 35,000 species of crustacean – seven times as many as there are of birds. Most prowl among the rocks and reefs – crabs, shrimps, prawns and lobsters. Some – the barnacles – have taken up a static life. Others – the krill which forms the food of whales – swim in vast shoals.
Robber crab (Birgus latro) climbing coconut tree, Aldabra Seychelles.
An external skeleton is highly versatile; it serves the tiny water flea as well as it does the giant Japanese spider crab that measures over three metres from claw to claw. Each crustacean species modifies the shape of its many paired legs for particular purposes. Those at the front may become pincers or claws; those in the middle, paddles, walking legs or tweezers. Some have feathery branches, gills through which oxygen is absorbed from the water. Others develop attachments so that they can carry eggs.
The limbs, which are tubular and jointed, are operated by internal muscles. These extend from the end of one section, along its length, to a prong from the next section which projects across the joint. When the muscle contracts between these two attachment points, the limb hinges. Such joints can only move in one plane, but crustaceans deal with that limitation by grouping two or three on a limb, sometimes close together, each working in a different plane so that the free end of the limb can move in a complete circle.
The external shell, however, gives the crustaceans the same problem as it gave the trilobites. It will not expand, and since it completely encloses their bodies, the only way they can grow is to shed it periodically. As the time for the moult approaches, the animal absorbs much of the calcium carbonate from its shell into its blood. It secretes a new, soft and wrinkled skin beneath the shell. The outgrown armour splits at the back and the animal pulls itself out, leaving the shell more or less complete, like a translucent ghost of its former self. Now, because the animal’s skin is soft, it must hide, but it grows fast and swells its body by absorbing water and stretching out the wrinkles of its new carapace. Gradually this hardens so the animal can again venture into a hostile world.
The hermit crab partly avoids this complicated and hazardous process by having a shell-less hind part and protecting it with a discarded mollusc shell, slipping into a new one in a minute or so whenever it has the need.
The external skeleton has one incidental quality which has had momentous results. Mechanically, it works almost as well on land as it does in water, so that, providing a creature can find a way of breathing, there is little to prevent it walking straight out of the sea and up the beach. Many crustaceans, indeed, have done so – sand shrimps and beach hoppers stay quite close to the sea; and pill bugs and penny sows have colonised moist ground throughout the land.
The most spectacular of all these land-living crustaceans is the robber crab. It is found on islands in the Indian Ocean and the western parts of the Pacific. At the back of its main carapace, at the junction with the first segment of its abdomen, there is an opening to an air chamber lined with moist puckered skin through which the animal absorbs oxygen. This monster is so big it can embrace the trunk of a palm tree between its outstretched legs. It climbs with ease, and once in the palm’s crest, cuts down with its gigantic pincers the young coconuts on which it feeds. It has to return to the sea to lay its eggs, but otherwise it is entirely at home on land.
Other descendants of the marine invertebrates have also left the water. Among the molluscs there are the snails and the shell-less slugs, but these emerged from water relatively recently in the group’s history. The first to make the move to land were probably descendants of the segmented worms, the millipedes. Their droppings have been found fossilised in the rocks of Shropshire. They were followed by pioneers which recent DNA studies show to have been crustaceans. And some of these made such a success of life in their new surroundings that they eventually gave rise to the most numerous and diverse group of all land animals – the insects.
There are few more barren places on earth than the plains surrounding a volcano in the aftermath of its eruption. Black tides of lava lie spilt over its flanks like slag from a furnace. Their momentum has gone but they still creak, and boulders still tumble as the flow settles. Steam hisses between the blocks of lava, caking the mouths of the vents with yellow sulphur. Pools of liquid mud, grey, yellow or blue, boiled by the subsiding heat from far below, bubble creamily. Otherwise all is still. No bush grows to give shelter from the scouring wind; no speck of green relieves the black surface of the empty ash plains.
This desolate landscape has been that of much of the earth for the greater part of its history. The first volcanoes to appear on the surface of the cooling planet erupted on a far greater scale than any that we know today, building entire mountain ranges of lava and ash. Over the millennia, the wind and rain destroyed them. Their rocks weathered and turned to clay and mud. Streams transported the debris, particle by particle, and strewed it over the seafloor beyond the margins of the land. As the deposits accumulated, they compacted into shales and sandstone.
