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00:00The End
00:30The creatures that live among the coral heads of the Great Ballya Reef in Australia must
00:58surely be among the most beautiful and the most bewildering organisms that you can find
01:03anywhere in the world.
01:28Sorting out all these creatures into their various groups is a baffling business and often
01:58things are not what they seem.
02:01These are the tentacles of a worm, but this is the cousin of a starfish.
02:08This is a flatworm and the creature advancing on it is a snail that has lost its shell.
02:15One thing is clear, they are all animals without backbones, invertebrates, but how are they related
02:20to one another?
02:21Which is descended from what?
02:24One way to try and find out is to trace the various groups as fossils back through the
02:29rocks to their origins.
02:38These limestones here in Morocco are so old, getting on for 600 million years old, that
02:47they date long before the time of any backboned animals.
02:51There are no fish fossils here, for example, but there are invertebrate fossils.
02:57Not as many or as varied, it's true, as the invertebrates that live today on the Ballya Reef,
03:03but invertebrates nonetheless.
03:05They fall roughly into three groups.
03:08There are little shells, like this, and a creature that looks like a flower but was covered in
03:15stony plates.
03:18And this, which is rather like a shrimp with a shell and its body divided into segments.
03:25What are the relationships between these three very, very early groups?
03:31If we could understand that, we would be close to understanding the origin of animal life itself.
03:37And the obvious place to look is a few feet farther down in these limestones, a million
03:42or so years earlier.
03:45But suddenly, we come to a mystery.
03:48Although these limestones look exactly the same as the limestones above and must have been
03:53laid down in very similar seas, there are no fossil shells to be found here at all.
03:59What's more, there are no fossil shells to be found in any rocks in the world of an age
04:04of these.
04:06And these extend for thousands of feet more, representing hundreds of millions of years
04:12of deposit, and not a fossil shell among them.
04:19The explanation is precisely in that word, shell.
04:23Shells fossilise easily.
04:25Soft animal tissues rot and hardly leave any trace behind.
04:30There was life in the seas in which these limestones were deposited, but it was without shells.
04:40But why did it take so long for animals to develop shells?
04:45After all, if you condense the whole history of life from its very beginnings until today
04:49into a year, it wasn't until early November that the first shelled animals appeared.
04:55Well, there's been a lot of debate on that question, and a lot of suggestions.
05:01One for example is that the chemistry of the seas wasn't suitable.
05:05They were either too cold or too acid to allow for the deposition of lime as shells.
05:12Whatever the answer, the fact remains that for that immense period of time, we have no fossil
05:18shells to help us chart the progress of the very early stages of animal life.
05:26But that doesn't mean to say that we can't make some informed speculations.
05:31For example, take this group of creatures, the one like little shells.
05:36What could their early ancestors have been like?
05:41These microscopic creatures are among the simplest animals in the sea.
05:45They're the larvae of corals and jellyfish.
05:48And we know that they appeared very early indeed.
05:52But suppose some of them didn't grow up either to float or to build skeletons, but took to
05:57a creeping life.
05:59They might easily have become something like this.
06:04This is a juvenile flatworm.
06:07It has a cluster of spots on top at one end, which are sensitive to light and to gravity.
06:12And it swims with the aid of cilia that cover its surface.
06:20When that settles on the sea bed, it becomes this.
06:24Not a drifter like a jellyfish, but an animal that moves in a purposeful way with a definite
06:29front end and back end.
06:41Flatworms are very flat.
06:43And with such a great body surface in relation to their small bulk, they're able to absorb
06:47all the oxygen they need directly through their beautifully patterned skin.
06:56Many of them move by rippling their bodies instead of relying entirely on the cilia.
07:08And some are so good at it that they can swim.
07:15A flat shape, however, is not so suited to burrowing.
07:32And as mud and sand began to spread over the sea floor about a thousand million years ago,
07:37rippling became a very desirable thing to do.
07:40There were bits of food to be sifted from the mud and hidden beneath it there was safety.
07:46So some worms changed from being flat to being round and long and buried themselves in the
07:52mud.
07:57Others were less active and remained with their front ends sticking out, ringed by tentacles.
08:07The beating of the cilia created currents that enabled the tentacles to absorb oxygen and
08:12also swept food particles down to the mouth at their centre.
08:17About 600 million years ago, some of these worms secreted a pair of shields on the top to protect
08:24the delicate tentacles and channel the feeding currents over them.
08:32This was such a success that variations appeared.
08:35The shells were strengthened with deposits of lime and grew bigger to give room for more
08:39efficient breathing tentacles.
08:40So eventually, the original worm-like shape was lost.
08:46We know from fossils that these creatures, the brachiopods, were enormously abundant in
08:51the ancient seas.
08:53They grew in many shapes and to a considerable size.
08:57Some developed delicate coils of lime inside their shells to support their feeding apparatus.
09:03But some 70 million years ago, their fortunes waned and today, only a few species survive.
09:10One lives in some numbers on the muddy shores of a bay in Japan.
09:16And at low tides, they're collected for food.
09:20They call them shamisen-gai because their shape is like that of the Japanese guitar, the shamisen.
09:40These are the simplest type of brachiopod that have outlasted all the more ambitious kinds that
09:47once were so abundant.
09:48In fact, they're virtually identical to those earliest fossil shells.
09:53It's an astounding example of survival and something that occurs several times in the history of life.
10:04An early species finds itself in surroundings which suit it to perfection.
10:09No other animal comes along later which exploits the surroundings any better.
10:14It's cousins may move away to colonize different environments or their own environments might
10:20change and so they develop into slightly different creatures.
10:24But this creature, encountering no change, sees no cause for change.
10:29And so it plods doggedly on an ultra-conservative.
10:33This formula of a simple worm-like body enclosed within a protective shell had obviously a lot of potential.
10:42Several groups of creatures in the very early periods were based on it.
10:46And one group in particular, the mollusks, exploited it very well indeed.
10:50Today there are something like 80,000 different species of them.
10:55The flatworm ancestors of the mollusks developed their shells not over one end around the mouth,
11:20but in the middle of the back.
11:22Originally, like a small conical tent under which the animal could hide, as the limpet does today.
11:30The shell is deposited by a fleshy part of the back called the mantle.
11:34And the animal enlarges it by adding to the margins.
11:40Some species though, don't do so at an equal rate all round.
11:44And that produces twists and coils in the shell.
11:52They have a well-developed head with eyes and sensory tentacles for feeding the whey and tasting the water.
12:12And underneath it, a very efficient feeding organ.
12:16It's a long tongue-like ribbon.
12:18The muscles around it press it down and pull it forward, rasping it over the surface on which the animal is crawling.
12:25And many species use it for eating algae.
12:35Looked at under the electron microscope, the reason for its efficiency is clear.
12:39It carries rows and rows of minute teeth.
12:42Each species, for some reason, with a different pattern.
12:45Cowries secrete their shell in a way all their own.
12:51They extend their filled mantle right round the shell and deposit material on the top.
12:57So giving it that beautifully polished surface.
12:59The spider shell has its ribbon tongue on a stalk.
13:14So it can scrape surfaces that its bulky shell would otherwise prevent it from reaching.
13:19It also has a stalked eye to help it prospect for such hidden pastures.
13:28Its foot has become very muscular to help it get around.
13:36Mollusks with paired shells, bivalves, don't often move far.
13:41Their foot is normally used to pull them down into the sand where they can sit and filter food safely and unobtrusively.
13:52The scallops are also filter feeders.
13:55They live on the surface and not only have good eyes, but a quite surprising way of getting around.
14:06Biggest of all is another filter feeder.
14:34The meter long giant clam.
14:37So huge it can't move.
14:39Its fleshy mantle joins its two shells forming a chamber beneath through which water is sucked.
14:46And every so often it gives a convulsive shudder and gets rid of a little waste.
14:52A few mollusks have gone to the other extreme and become free swimming by reducing their shells to mere scales concealed within their bodies or doing without them altogether.
15:08Unprotected by a shell, these creatures defend themselves with a nasty tasting slime.
15:26And their brilliant colours may serve to warn off anything that might contemplate making a meal of them.
15:32If that's so, they must be among the loveliest warning notices in all nature.
15:38In this sort of glories there are just and by little things that might look a little like that.
15:43In this type of piousness, this is the message has been nailed down.
16:18These creatures are more complex and usually larger than flatworms, and they need special
16:27breathing apparatus, the gills.
16:30In some species, they're exposed as a kind of trembling bouquet at the back.
16:48. . .
17:06Several kinds have developed feathery outgrowths that enable them to float close to the surface
17:13of the sea.
17:14And there, extraordinary though it may sound, they hunt for jellyfish.
17:22This one is called Glaucus, and it has found its prey.
17:28. . .
17:29The stinging cells of the jellyfish are no defence.
17:35Indeed, some of these floating mollusks welcome them, swallowing the stinging cells and storing
17:40them in their own tentacles to use as second-hand weapons.
17:45. . .
17:54This is another creature eaten by Glaucus, one of the most poisonous and deadly of all jellyfish,
17:59the Portuguese man-of-war.
18:01Beneath it trail its tentacles, loaded with stings.
18:05Another mollusk also preys on this creature, and this time one with a shell.
18:10It has a most ingenious solution to the problem of keeping afloat.
18:14. . .
18:16It produces bubbles by trapping air in mucus with special movements of its spoon-like foot,
18:23and builds them into a raft from which it hangs.
18:26. . . .
18:41When it drifts into a Portuguese man-of-war, it attacks immediately.
18:46. . .
18:55The stinging cells of the jellyfish, lethal to other creatures, have no effect on the snail.
19:17It munches them with the rest of the tentacles.
19:20. . .
19:26A raft of bubbles solved this snail's weight problems, but that device won't work for bigger creatures.
19:33. . .
19:34500 million years ago, however, a group of mollusks evolved another method.
19:39. . .
19:41This fossil shell may look perhaps quite an ordinary sort of shell, albeit rather large,
19:48. . .
19:49But inside, it has got quite a complicated structure.
19:52. . .
19:53Here's one in a boulder where the outside has been worn away so that we can see what's inside.
19:59. . .
20:00và o mouth was where the animal itself lived, and at the back of it there were these chambers,
20:06. . .
20:09, which in life were filled with gas and acted as flotation chambers.
20:12. . .
20:13can we be so sure? Well, because this is another of those creatures that have survived virtually
20:21unchanged for hundreds of millions of years. This is a nautilus, and there are nautilus
20:28swimming in the seas today. They live in the South Pacific, but few people ever see them
20:36alive, for they spend most of their time in depths of up to 500 metres. But they can
20:42swim at any depth by pumping fluid in and out of their inner chambers, and so controlling
20:47their buoyancy. Being so mobile, they need good sense organs, and their eyes, although
21:03they don't have lenses, are the best of any creature that we've seen so far. Their bodies
21:09have become modified into dozens of tentacles. Some carry sense organs to detect food, some
21:15are used in reproduction, and others to grapple with their prey, which is usually either carrion
21:21or lobsters or crabs.
21:28This proved to be an immensely successful design, and from it came another great group of mollusks,
21:43the ammonites. The ammonites were to dominate the seas of the world for the next 200 million years,
21:49and they left behind in the rocks, particularly here in Lyme Regis, in southern England, fossils
21:55that, to my mind, are some of the loveliest fossils of all.
21:56Like the nautilus, the ammonites added new flotation chambers as they grew, while their bodies occupied
22:07only the outer one.
22:11Because ammonites were so numerous, and their shells fossilized so well, we know a great deal
22:20about the way they developed over a period of 200 million years. But their history is full
22:26of puzzles. Why, for example, did some groups develop uncoiled species, and then, over generations,
22:33slowly coil up again?
22:42And why did the junctions between the flotation chambers, which originally had been simple curves,
22:48become increasingly elaborate, and intricate, and eventually florid?
22:55Small ones may have lived in shallow water near the bottom,
23:02but others grew to an immense size, and probably sailed the upper waters of the prehistoric seas
23:09like galleons. And there is one final mystery. Why, 50 million years ago, did they all die
23:16out? There is not one surviving ammonite today.
23:25But these paper-thin shells look remarkably like them. On very rare occasions, they're washed
23:30up on lonely beaches in New Zealand. They belong neither to an ammonite nor a nautilus,
23:35but to a relative of both, a kind of octopus called the argonaut, which is sometimes stranded
23:40with them. The animal doesn't live in the shell. It secretes it from one of its arms, and then
23:46lays its eggs in it. Few people have ever seen that happen. Just once in a while, a storm
23:52catches the breeding shells and drives these delicate cradles ashore, some of them still
23:57holding their eggs. For most of its life, the argonaut, like all other octopus, is totally
24:03without a shell. Only on this one occasion does it demonstrate its relationship with the
24:09nautilus so vividly.
24:18It's difficult to remember at times that the octopus is a mollusk, and that most of its
24:22relations are weighed down with shells, and a very long way from being quick-moving or
24:27intelligent. Its molluscan tentacles have become heavily armoured with suckers. The siphon,
24:34used by the clams for filter feeding, serves as a very effective nozzle for jet propulsion.
24:40Its eyesight is excellent, and the animal has a lively brain and very quick reactions.
24:47The squid is very similar. It has two more arms than the octopus, and is a very much more
24:53active swimmer.
24:56Squids still keep within their bodies a last relic of their ancestral shell, a horny sword-shaped
25:02structure that helps to support their long body. As they swim, they hold their tentacles
25:07out horizontally. They use jet propulsion for speed, but they can also idle along in either
25:13direction by waving fin-like extensions of their mantle.
25:32The squids and octopuses are the most active and intelligent of all the mollusks, able to solve
25:39quite complicated problems. They're also the largest.
25:47This giant squid that ran aground in Norway was nine metres long, and there are reports of
25:52others twice the size. And they all developed from ancestors like flatworms that lived in the seas of 600 million years ago.
26:01But what about the second group of creatures from these very ancient rocks?
26:08The ones represented by this flower-like fossil with a radial symmetry?
26:13Well, within the next few million years, these developed into a multitude of most beautiful forms
26:18that we call sea lilies or crinoids.
26:23It's a reasonable guess that these two evolved from worm-like creatures of some sort that developed limy plates to strengthen and protect themselves.
26:32These are about 300 million years old, and a very few species like them still survive in the ocean depths.
26:42But on the barrier reef, some close relatives still flourish in great numbers. Feather stars.
26:49These are just like crinoids, but without stems, except when they're very young.
26:55These adults swim freely around, mostly at night, in search of good feeding places where they can cling to the rocks and collect floating particles with their arms.
27:06Their relatives, the starfish, show particularly clearly another characteristic of this group.
27:20Their bodies have a five-fold symmetry. The mouth is underneath, at the centre.
27:26They move on tube feet, another unique feature. Each foot has a tiny suction pad at the end, and the many thousands of them are worked by hydraulics,
27:36for they're all connected to water-filled vessels that run through the body.
27:45Their cousins, the brittle stars, are much the speediest creatures in the whole group.
27:50Sea urchins are more typical. They, too, have tube feet, but they move largely with the help of their spines.
28:04Some of the tube feet are specialised for particular jobs, such as moving bits of debris from around the mouth,
28:10which, like that of the starfish, is on the underside of the animal.
28:14Urchins feed by grazing slowly on algae.
28:21The food is gnawed by hard jaws, taken into the gut, and then, in most species, excreted from a paw at the top.
28:31The spines are attached to the plates of the urchin's shell by ball and socket joints, so that they can move in any direction.
28:37Those on the top are primarily for defence. If a shadow falls on the urchin, it swivels its spines quickly to point towards a possible attacker.
28:47These creatures may seem to be very different from the original crinoids, but they all have a radial symmetry and tube feet.
28:55And although we can't be sure of evolutionary pathways, relationships can be plainly seen.
29:00If the head of a crinoid drops on its face, it becomes a starfish.
29:07This, thinned down, turns into a brittle star.
29:11But if it thickens, curls its arms back on itself and grows spines, it becomes a sea urchin.
29:17One group became elongated and lay down on its side to feed.
29:22It's obvious why it's called a sea cucumber.
29:25Most of these creatures work their way over the sea floor, feeding on detritus, a pretty nondescript sort of animal, you might think.
29:37But their tube feet give the clue to their true relationship.
29:40This whole group of hydraulically driven creatures hasn't produced any swift-moving, highly intelligent forms.
29:56But in their own terms, they've had a solid success.
29:59There are about 5,000 species of them.
30:02And whenever there's a suitable opportunity, they miraculously appear, often in great numbers.
30:06The crown-of-thorn starfish is normally quite uncommon.
30:10But periodically, thousands materialise on a reef and start to eat the coral.
30:16The secret of the group's success lies in their larvae.
30:25Too small to be noticed by the naked eye, these larvae swim in millions in the sea.
30:30This will eventually become a starfish.
30:45And this, similar in many ways, turns into a sea cucumber.
30:51Nearly all marine invertebrates, mollusks, sea urchins, worms, corals, jellyfish, all reproduce by larval forms like these,
31:02which are swept by the currents into every part of the oceans.
31:06The vast majority will be eaten by fish.
31:09Great numbers fail to find a suitable home and simply die and dissolve into nothing.
31:13But their presence everywhere ensures that no suitable corner goes unoccupied.
31:22The larval sea snails have to support the extra weight of their tiny developing shells with lobes covered by beating cilia.
31:29The similarities between larval forms are just as valid evidence of relationship as those between adults.
31:50And the fact that this mollusk larva looks very like this, the larva of a segmented worm,
31:57is a strong indication that the two groups are descended from a common ancestor.
32:05Eventually, this larva becomes a worm such as this,
32:10the simplest member of our third group of animals, the segmented ones.
32:13These worms probably developed a series of segments in their bodies, each with its own pair of moveable bristles,
32:20because it made sustained burrowing easier.
32:25Soft-bodied animals hardly ever fossilise, but in one exceptional site in South Australia,
32:31in rocks 650 million years old, even older than those limestones in Morocco,
32:37have been found what appear to be segmented worms.
32:39This is one of the very earliest records of a soft-bodied animal that has ever been found.
32:54There is one other highly exceptional fossil site where soft-bodies have left their impressions in the rocks.
33:00It lies in the heart of the Rocky Mountains in British Columbia.
33:03In the rocks here, you can get a unique glimpse of the kind of animals that crawled around the bottom of the seas
33:13a hundred million years after those very early Australian ones,
33:17in fact, at just about the same time as those in Morocco.
33:20These rocks are shales, mudstones, and of the very finest texture.
33:36From a detailed examination of them, we can be pretty sure that they were laid down at the bottom of a sea about 500 feet deep.
33:44But this particular patch was a very special one.
33:49There were virtually no currents, and in consequence, no oxygen.
33:54That meant that no creatures could actually live in this little part of the sea bottom.
34:00There were no scavenging animals, for example.
34:02And equally, there was no oxygen to fuel the processes of decay.
34:09So that meant that if any dead creatures drifted down to settle on these muds,
34:17their bodies would remain intact for a very long time.
34:21And come they did.
34:25Fine mud settled on top of them, and so they were entombed.
34:30Over millions of years, the mud consolidated to form these shales.
34:37And here, as fossils, they have remained,
34:40miraculously escaping the distortions and crushings that happened
34:45when these rocks were rocked up by earth movements to form the Rocky Mountains.
34:50And these freak conditions have preserved the most delicate of creatures.
34:56Here, for example, is a little worm.
35:00Several species of segmented worms have been found,
35:10and their preservation is so remarkable that you can almost count their bristles.
35:14There's also a group of creatures that, while they seem to be related to the segmented worms and are rather more complex than they are,
35:28are nonetheless quite unlike any creatures that are alive today or any other later fossils that we know of.
35:44You might call them experiments in animal design, experiments that didn't quite come off.
35:51They weren't quite efficient enough to survive in the battle for living that was becoming increasingly intense.
35:56Look at this one, for example.
35:59It appears to have seven pairs of supports, and above each, a tentacle with its own mouth.
36:05Even compared with some of today's strange creatures, it seems grotesque and outlandish.
36:13This five-eyed creature has a long trunk here bent back along its body.
36:19It was probably used for detecting and manipulating food.
36:26This one is of particular interest, for it has stumpy little legs down each side.
36:31And in this case, there does seem to be a close living parallel.
36:35It's not a sea creature, but one that lives in moist jungles.
36:38Peripetus.
36:53Clearly, segmentation was a great evolutionary success.
36:57The appendages on each segment becoming more and more specialised as legs and gills and mouth parts.
37:03Some of the commonest fossils here are trilobites, like the one we saw in Morocco.
37:12These had hard shells, part calcium carbonate and part chitin.
37:17And they fossilised well all over the world, for they swarmed everywhere in the seas of four to five hundred million years ago,
37:24during November in our life on Earth year.
37:26Because their body armour was not expandable, the trilobites had to shed their shells regularly in order to grow.
37:40Indeed, many trilobite fossils are of these discarded shells.
37:45Sometimes they occur in great drifts.
37:47Here, almost entirely the tail ends, presumably sorted out by the sea currents, as shells often are today.
37:58When the complete animal has been fossilised, we can see from a variety of positions that some trilobites could roll up for protection, like wood lice today.
38:07More information can be discovered by making x-rays of some particularly perfect fossils.
38:14They even reveal details of the gut and muscle fibres inside the animal's body.
38:20But perhaps the most astounding thing about trilobite fossils is the preservation of their eyes.
38:27Although our knowledge of the internal structure of their eyes limited, the hard parts, the outer lens system, is often fossilised in superb detail.
38:37Even the earliest trilobites we know had compound eyes, each element providing a part of a mosaic picture,
38:45which in this species, at any rate, gave the animal an almost spherical field of view.
38:49If the fossil eye is sliced, it's possible to discover how each lens was constructed.
38:56It was a single crystal of calcite, lined up in such a way as to give the clearest image.
39:02There could be several thousand in each eye.
39:06Later in their history, some kinds of trilobites evolved even more sophisticated eyes.
39:11Here the lenses are less numerous but larger, and it's thought that each provided a separate, well-defined image instead of a mosaic.
39:21By slicing one of these fossilised lenses, a remarkable discovery has been made.
39:26The lens is really a doublet, that is, it has an upper and a lower element.
39:31This is the line of their contact.
39:33It's almost identical with the design recommended by mathematicians in the 17th century for correcting spherical aberration in thick lenses.
39:43Evolution solved the problem for the trilobites 400 million years before man.
39:49The doughnut shapes of the lower lens elements have been preserved alone in these fossil eyes.
39:56In most cases, it's the upper lenses that can be seen.
39:59Although trilobites possessed the first sophisticated optical system on Earth, some species were blind.
40:08They must have inhabited dark, muddy waters where there was no light and no need for eyes.
40:16The great variety of size and shape in trilobites suggests that they had a wide range of habits.
40:23It's probable that some scavenged their living on the muddy bottom,
40:26whilst others were quite active swimmer hunters.
40:30But finally, some 250 million years ago, their great dynasty came to an end.
40:35Though one relative managed somehow to hang on.
40:40This is it. The horseshoe crab.
40:42It's sufficiently different from a trilobite with this very big head shield for us to put in a group on its own.
40:59But it's also sufficiently similar for us to be pretty sure that the two groups are closely related.
41:03It's got a pair of these eyes on the front, which are mosaic eyes, very like those of a trilobite.
41:12And underneath, it's got a segmented body with a pair of legs on each segment.
41:19And at the front, a fist with a hook on it.
41:26That is a sign that this is a fully mature male, because it uses that in breeding.
41:33On a few nights in the spring, when the moon and the tides are just right, and this is one of them,
41:43these antique animals crawl up out of the sea to nest here on the beach.
41:49This male is one of the advance guard.
41:52But as the night wears on, there should be hundreds and thousands of them.
41:56Horseshoe crabs are found all along the eastern seaboard of North America.
42:19But this beach in Delaware Bay is the best place to see them in large numbers.
42:23Here, at least, we can get some idea of what things may have been like
42:28when their distant relatives, the trilobites, swarmed in the seas of long ago.
42:32It's been an inc
42:39this particular trip of long ago.
42:45It's been an incredible нак
42:49like the eagle that has lost its clutch.
42:53It has been very subtle as the irreederacy of the tree.
42:56At the center of each mass is a large female, and directly behind her, attached by his claws,
43:12is a male who will fertilize the eggs.
43:15Other unsuccessful males crowd around.
43:17The egg mass is laid several inches down in the sand, and remains there while the tiny larvae develop inside.
43:31Because of their shape at this early stage, they're known as trilobite larvae.
43:36At the next high tide, about a month after the eggs were laid, the sea reaches them again.
43:53The eggs rupture, and the larvae swim free.
43:57Thousands will get eaten within hours, but a few will survive to continue this very ancient line.
44:06Swimming with them in the plankton are creatures related to those segmented animals in the British Columbian shales.
44:15These survived unobtrusively throughout the reign of the trilobites, but have since come into their own, the crustaceans.
44:23This is one of them, a copepod, with its remarkable simple eye.
44:27But there are about 3,500 different species of crustacean today.
44:32Most of them have adopted a floating way of life, and are the staple food of many kinds of fish, as well as of whales.
44:46They have many lifestyles.
44:49Some of them are completely unknown.
44:52This creature has, on a number of times, been seen holding a tiny jellyfish.
44:56Is it using the jellyfish's stinging cells as protection?
45:02Or is there some other relationship between the two?
45:07Whatever their way of life, all crustaceans have one problem in common.
45:11The same one that the trilobites had.
45:14Their external skeleton won't expand.
45:17So if the animal is to grow, it must be shed.
45:19First, it extracts some of the important salts from its skeleton, and re-absorbs them into its bloodstream.
45:29Then, it begins to molt.
45:30MUSIC PLAYS
45:38It's new skeleton is soft and crumpled, but it quickly expands.
46:05For a while, the animal is vulnerable, but as the salts are slowly fed back into it, the shell hardens.
46:19In spite of its problems, an external skeleton, as developed by the crustaceans, is clearly
46:24a very effective and efficient way of building a body.
46:28And nothing could demonstrate its potential better than creatures that live in this bay
46:33off the coast of Japan.
46:35Because down on the sea bottom, 600 metres down, there live the largest crabs in the world.
46:41And this boat is fishing for them right now.
46:43.
46:50.
47:11Without the support of water, its long legs flop.
47:14The muscles are simply not strong enough to hold them rigid in air.
47:18.
47:35Each leg is a tube down which a strand of muscle runs.
47:40The muscle is attached to a projection from the next joint so that when the muscle contracts,
47:45the joint moves.
47:47It's rather like the arm of an industrial crane which has an outer network of steel girders
47:52down which a wire hawser runs.
47:55Of course, the one sort of joint you can't put on such a system is a ball and socket joint
48:01which gives the sort of universal movement that I can have in my shoulder or my thigh.
48:06But the crab deals with that by having each of these joints working in different planes.
48:11So that one way or another, it can reach almost anything that is within its immediate neighbourhood
48:18and convey it with its pincers to the mouth where it's chewed up in these mouth parts.
48:23Its body is protected by this heavy armour of shell and the crab can tell what's going on around it
48:31because through the armour they project tiny little sensory bristles.
48:35This creature is indeed spectacular.
48:37But every now and again from the waters of this bay, the fishermen bring up a real giant.
48:43And creatures like this are over 11 feet across.
48:50Most crustaceans, however, are of a more modest size.
48:55Apart from the myriads of tiny ones in the ocean,
48:58there are vast numbers of small crabs and prawns and shrimps,
49:02all with specialised ways of life.
49:05All these, for example, come from just one small patch of the Great Barrier Reef.
49:18Crustaceans use pigments for camouflage in the most elegant way.
49:32Some, in fact, are very difficult to see at all unless photographed in close-up.
49:44The crustaceans show clearly what advantages can come from having a body divided into segments.
49:50Each can bear appendages and the crustaceans have modified them into many different tools.
49:56Sometimes they're used for respiration, sometimes for reproduction,
50:06some as antennae, mouthparts, food manipulators, pincers, and, of course, legs.
50:26An external jointed skeleton has got one quality that I've not yet mentioned.
50:35Mechanically, it works just as well on land as it does in water.
50:40So, from that point of view, you might say that the crustaceans are pre-adapted to life on land.
50:45And, indeed, one group has made the move.
50:48And quite formidable animals they are, too.
50:51This is a rubber crab.
50:54I must handle him with some care because you can get quite a nip from these pincers.
51:02He uses them to cut down young coconuts on which it feeds.
51:07And it's said that he can even hammer a hole into a mature coconut,
51:11though no one's actually seen him do it.
51:14He breathes through a chamber at the back of the shell here.
51:21It doesn't contain gills, but the oxygen is absorbed through the puckered lining of the chamber.
51:27So, here is a creature that can breathe on land, move on land, eat on land.
51:33It's true it has to go back into the sea in order to breed, but otherwise it's a fully operational land-living animal.
51:41Other descendants of sea-living invertebrates have also made the move onto land at various times.
51:58There are snails, for example.
52:00Though robbed of the support of water, they're never able to grow their shells on land as big as they do in the sea.
52:06No, it's the segmented animals that have adapted best to land.
52:11And of all those, it's the ones that did it first, who have been most spectacularly successful.
52:17The insects.
52:19They emerged some 400 million years ago, and they wrote the next great chapter in the history of life on Earth.
52:27The fire has the same.
52:39Everyone is on earth.
52:42The six of the plants are now and there are no plants that have grown in the end.
52:47¶¶
53:17¶¶
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