Survivors: The Animals and Plants that Time has Left Behind. Richard Fortey

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      Poking about some more in the rotten wood we make another discovery: baby velvet worms. They are only about one centimetre long, and pale in colour, but they seem to be exact small versions of the large ones. I presume they must eat suitably diminutive termites. The worms grow continuously to achieve adult size, blowing up like balloons. The velvet worm actually gives birth to live young, and the ones we saw may have been newly born. This is unusual among invertebrates, and even among vertebrates is only characteristic of mammals (and a few specialised reptiles). The eggs of this particular velvet worm are few in number, and large and yolky, thus allowing for further embryonic development within the female; only three or four young are born at one time. There are two ‘litters’ a year. Since the animals live for three years they only have about twenty offspring, which is an extraordinarily small number when one remembers that most arthropods, for example, lay thousands of eggs: recall our horseshoe crabs. The most prolific velvet worm species produces no more than forty young a year. It seems that Peripatus is an animal with a personality all its own.

      Looking a little more closely at the velvet worm, the first thing to notice is that the body seems to be made out of many rings that encircle the body, even the legs and antennae. They remind me of the Michelin Man, supreme advertising logo of the famous tyre company, all dressed up in his bands of rubber. It is this distinctive structure that accounts for the body’s elastic properties. Muscles circle the body cavity inside the skin. Then it is obvious that this is a segmented animal rather like a trilobite, with lots of similar units repeated along the length of the body. Each body segment carries a pair of those stumpy legs. Among living species of velvet worms the number of segments varies quite widely, but, biologically speaking, that is only a matter of tacking on extra identical units, and does not require massive tinkering at the genetic level. To prove this, there is even one velvet worm species that can have between twenty-nine and forty-three body segments. The short, stumpy legs propel the animal along by working in sequence in waves, a common feature among segmented animals. From the side, it looks as if one leg hands on a motion to its neighbour progressively in a common direction. Forward movement would obviously not be possible if legs pushed forwards entirely at random; cooperation is required. The legs remind me of the limbs of a child’s stuffed toy, rather like those belonging to Piglet as illustrated by E. H. Shepard in The House at Pooh Corner, but they work well enough to catch up with termites. After all, one does not need a Maserati to overtake a donkey. Looking more closely at the surface of the ‘skin’ each of the body rings carries a line of protuberances, giving the external surface a knobbly appearance, especially on its upper side – these are known as papillae (they may have even smaller secondary papillae upon them). The patterns of the papillae vary between velvet worm species, as does the overall colour. There is one magnificently blue species elsewhere in New Zealand.

      The head of Peripatus is most obviously identified by its pair of antennae. But close to the front on the underside is the mouth, which is provided with sickle-like jaws to either side, each equipped with a pair of blades at the tip that are produced by a local thickening of the skin, or cuticle. They are simple but efficient shredders. The ducts for the slime glands open at the side of the head. There are no eyes. As for the legs, they are little more than stumpy projections off the body equipped with muscles internally to swing them backwards and forwards. Their feet carry two sickle-shaped claws at their tips, which are much like the jaws in structure; this may indeed provide a clue to the evolutionary origins of the more specialised jaw. Males and females are similar, except that the former are usually a little smaller and are less common.

      Inside they are pretty simple, too. The major part of the body is taken up with the stomach, which runs along the length of the animal to the anus at the end. Between the gut and the mouth there is a short oesophagus and a muscular pharynx, which is used for initial food processing. Oxygen absorption is achieved through tiny tubes inside the body called tracheae that have their apertures located in depressions between the papillae. There are no special gills or lungs because animals of this size can get all the oxygen they need through thinned parts of the cuticle. The heart is another simple tube, positioned at the top of the body above the stomach. The rest of the vascular system is much as in the horseshoe crab Limulus, distributed rather diffusely through the internal cavity. Peripatus gets rid of its waste products by means of nephridia, kidney-like organs, located in the legs along with small excretory openings. The nerve cord is a double structure running along most of the length of the animal, with cross connections that make it look somewhat like a ladder: nerves extend from this into the segments and limbs. A larger ganglion in the head is all that this basic creature can display as a brain.

      Simple though it may be, the velvet worm functions perfectly well. For a moving animal, there is quite a short list of vital functions: sensory equipment to find a source of food and tools to help eat it; a method of locomotion; a way to breathe and distribute oxygen to internal organs; a system of waste disposal; a reliable way to propagate the species. Peripatus would be the kind of creature one might put together from a ‘how to make an animal’ kit, except that like almost everything else in nature it has some tricks all its own – its gluey trap, its ability to produce little peripati by live birth. It is a simple creature in many features, specialised in other subtle ways; but it is also another old timer, a messenger from the distant past.

      Its more recent history is not very different from that of the podocarp trees. Peripatus and its relatives number about two hundred living species (placed together in Phylum Onychophora, informally known as ‘onychophorans’).^* They also have a distribution over the areas that once formed Gondwana: Australia, New Zealand, South Africa, South America and Assam (India). There are also velvet worms in Irian Jaya and New Guinea, where it is very likely that further species still remain to be discovered in mountainous and inaccessible areas. All of them carry the long memory of the vanished supercontinent as they tramp their unadventurous way on their stubby legs. Velvet worms had once wandered over Gondwana but, like the podocarps and southern beeches, new species arose on the separate pieces of the progressively fragmented continent; for evolution does not stand still. I could have gone in search of the velvet worm in any one of these other regions. The New Zealand species I happened to pursue is particularly interesting because it has developed a relationship with a special kind of termite that is regarded as the most primitive of its kind (of the Family Kalotermitidae), among which most individuals finish up as flying insects. The other termites are noted for their extraordinary caste system, with specialist workers and soldiers that never change their roles. It seems possible that a whole primitive ecology was transferred to New Zealand when Gondwana broke up, and there it endured, virtually unchanged, encased in logs. But something did change. I found Peripatus inside a pine log belonging to a species that was not native to New Zealand, so at some recent date the velvet worm must have followed its termite prey into a new habitat. You can teach old worms new tricks.

      Since the velvet worm has a body as soft as dough it is most unlikely to be preserved as a fossil. Shells and bones leave behind their hard evidence, but can we expect a shy, soft package of flesh to do the same? Even the Solnhofen Limestone fails to preserve a single fossil of a Peripatus. Fortunately, there is one example preserved in Cretaceous amber from Burma (Myanmar), perhaps 100 million years old, a contemporary of the dinosaurs. Amber preserves the most evanescent of creatures: flies, beetles, even mushrooms. This fossil species is very like the living Peripatus and there is no question that it lived in a similar fashion. It provides the proof that velvet worms of modern type were alive at the break-up of the Gondwana supercontinent – which is good to know although we might infer it anyway from their distribution today. But we want to go back much further than this, 200 million years earlier. A remarkably preserved impression from the Carboniferous called Helenodora tells us that in the swamps of the coal measures, distant relatives of the velvet worm – but still eminently recognisable – were wandering their deliberate way through the damp undergrowth. Their contemporaries at this stage in the evolution of life were inelegant amphibians and very early reptiles, accompanied by the first flying insects. The velvet worm was terrestrial then, just as it is now. It may even have developed its special slimy-gluey glands, although at this early date it must have fed on something other than termites: for these insects were not even a twinkle in the eye of evolution in the Carboniferous. The velvet worm is beginning to look at least as ancient as the horseshoe crab. The velvet worm likewise survived the great extinction