ZOO 138, Monday, February 3, 1997, 12:00 CLASSIFICATION AND NATURAL HISTORYOF THE AMPHIBIA
We're running late and it's not a good place to .be. I'm going to postpone my completionof the lecture on the Invasion of Land for a while. The reason for that is that the third area that weneed to talk about of the Invasion of Land was constancy of the temperature.
And I have a lecture scheduled where I'm going to talk about the solution to thatproblem, which has to do with the regulation of body temperature, and difference between reptileson one hand and birds and mammals on the other.
So when I give that lecture, I'll go back to that issue of constancy of temperature andcover that material. that I was going to cover there. Instead I'm going to go onto the classificationand natural history of the amphibian, and this is included in your Illustrated Notes.
And what you can see is that the class Amphibia, which is in the superclassGnathostomata as will be true of the reptiles and vertebrates that I talk about.
The class Amphibia is divided into 2 subclasses.
An extinct subclass called labyrinthodontia. And a subclass called Lissamphibia thatincludes all three of the living orders of amphibians.
So today I'm going to be giving a little bit of introduction to the Labyrinthodonts, andthen talking more about the living orders of amphibians In your Illustrated Notes you have afamily tree of the Amphibia. And you can see that there are all these little sort the squiggley lineswith little names associated with them, Ophidia and sauropleura and so forth. You do not need topay any attention to all those little names there.
All those little squiggley lines are there as to sort of graphically communicate to you thatthis group of animals, the labyrinthodontia which are represented by those names and lines, was avery diverse group. It was around for quite a long time. You can see that they first appear in thelate Devonian. And they don't die out until the end of the Triassic.
They are around for something like 400 million years ago to around 120 million yearsago. So they were around for a very, very long time. And they were very diverse. And they gaverise not only to the modern amphibians but also to the reptiles.
And the other piece of evidence that's sort of presented graphically here is that the lines ofevolution that gave rise to the 3 living groups of amphibians, the frogs, a group of animals thatyou probably haven't heard of that are the limbless types, and the salamanders.
Those 3 lines of evolution are -- go all the way back into the labyrinthodontia. It's notreally clear which groups of Labyrinthodonts gave rise to the amphibians.
There is some evidence that they may be descended from a single group, because theyshare a number of characteristics. But there is also some lines of evidence that somepaleontologists have interpreted from the fossil record and they say these animals have differentancestors from the Labyrinthodonts.
So that diagram represents 2 ideas. There were a lot of Labyrinthodonts, they were verydiverse, they were around for a long time. And secondly, we don't know which group in particulargave rise to the living amphibians.
The next page is sort of a schematic representation of what a Labyrinthodonts lookedlike. Some of them were pretty good sized, several meters in length. They probably looked sort oflike a large reptile to us. They had scales in their skin like a large reptile.
And the name "labyrinthodontia" is a reference to the structure of their teeth which, is afairly peculiar tooth structure. You have you a diagram there of sort of a cutaway side view ofwhat one of their teeth looked like. From the outside, the teeth appeared to have a. whole seriesof lines. In other words, if you looked at that tooth just from the outside, what you see is a wholeseries of grooves in the surface of the tooth.
If you cut through the tooth, what you see is that the outer layer of the tooth, which is thesame material that forms the outer layer of your teeth, that's enamel. The enamel was very highlyfolded. This is the outer layer, that's the enamel.
And it is folded producing the appearance of these grooves on the outside of the tube.And to some imaginative paleontologist this looked like a maze or a labyrinth. And so this istermed "labyrinthine dentition." Labyrinthine dentition is the term that describes the teeth in theLabyrinthodonts. And refers to this highly folded outer later of the tooth, the enamel.
There are 2 other groups of vertebrate animals that have labyrinthine dentition. Onegroup is the rhipidistia crossopterygii. The rhipidistians have labyrinthine dentition.
The Labyrinthodonts, the ones I'm talking about right now, have labyrinthine dentition.And a third group. which are reptiles in the order Captorhinomorphs, is the third group ofvertebrate animals that have labyrinthine dentition. We have a group of fish, the crossopterygii.We have a group of amphibians. And we have group of reptiles.
Now, whenever we see similar structures or similar physiology or even similar behaviorsin one or more or in say in 2 different groups of animals or 2 or more different groups of animals,there are 2 possible interpretations. Two ways in we can interpret the. presence of that similarcharacteristic in the different animals.
Either they have this characteristic because they all inherited it, either from some commonancestor or from one another. So the similarity can be due to inheritance. Or the similarity canmean that they each came up with it independently.
And if they each came up with it independently, that means that they started fromsomeplace that was different and ended up in the same place. They all start off with a differenttooth structure and they ended up with the same tooth structure. And term that we would use forthat would be convergence, coming together, convergent evolution.
We are going to have many examples of convergent evolution in here in the next week orso, places where different types of animals have ended up looking or acting or having similarphysiology, having started from some place else and they come together, hence, the termconvergent evolution.
So we have these 2 choices: Is this an example of similarity due to common ancestry or isthis an example of similarity due to convergent evolution. Convergent evolution is going to be theresult of animals adapting to a similar set of conditions.
We're going to see a group of reptiles that existed during the Jurassic that look a lot likedolphins, and that's going to be interpreted as an example of convergent evolution, because theyhave adapted to life in the ocean, catching fish, they have stream-lined bodies, they have tails havebeen enlarged into something that will propel them through the water, and so forth.
So faced with that dilemma is do you interpret the similarities between these animals asbeing due to convergence or due to common ancestry? .
The standard is to take the simplest explanation. That's sometimes called Ocam's(spelling) razor, but the idea is that you are going to take the simplest explanation. And thesimplest explanation, unless there is some evidence to the contrary, the simplest explanation isthat it's due to common ancestry.
So the interpretation of this labyrinthine dentition is that the crossopterygiians, which isspecifically the Rhipidistians evolved into the Labyrinthodonts, and Labyrinthodonts evolved intothe. Captorhinomorphs because these animals are found in similar habitats but in differentgeological periods.
So the Labyrinthodonts have labyrinthine dentition, because they inherited it from theRhipidistia. And the Captorhinomorphs have labyrinthine dentition because they inherited fromthe Labyrinthodonts.
Now, there are some other interesting evolutionary things that occur in theLabyrinthodonts. One is that they have elaborated the vertebra, the fishy vertebra of theRhipidistian is not even a completely solid bony structure. And by the time we get to theCaptorhinomorphs we see that the vertebra itself has become a large solid and substantial bonystructure. So that's one of the evolutionary changes that came about.
Among the Labyrinthodonts, what we see is a variety of different solutions to thequestion of how many fingers and toes do you want to have. There are some Labyrinthodonts thathave 7, some that have 8, at least one that had 8. But the ones that gave rise to the. reptiles hadfive.
And so that's why you have five fingers and five toes, and why almost all vertebrateseither of five fingers and toes or can be seen to have reduced it down from five to something less.A horse has only one. A cow has 2. But the sort of early decision, which may not have been basedupon anything particularly adaptive among the Labyrinthodonts was that they settled on five,that's why the rest of the vertebrates have five.
So those are the Labyrinthodonts. They are really interesting, and an important group inan evolutionary sense. They were very diverse. They were living in different habitats, and theywere doing all kinds of things in an evolutionary sense, sort of evolutionary experiments withdifferent structures of the vertebrae. But the ones that gave rise to the rest of the reptiles came upwith this system that we see in the reptiles with the structure of the vertebrate that they have.
The other subclass which includes all of these living forms is the subclass Lissamphibia.And as I said, there are differences of opinion as to whether or not these animals have alldescended from some common ancestor or have through convergence have evolved a similar setof features. But, nevertheless, they do have a set of characteristics which distinguish them andwhich they share with one another.
One feature has to do with teeth, but it probably doesn't have anything to do withlabyrinthine dentition. It seems to something that's a completely different kind of phenomenon.And that is they have teeth that have a. separate little bony foundation down within the jaw.
It's shown here on the bottom of the page. But if you look at the jaw and your tooth, forexample, your teeth are set a solid structure that sets down inside the jaw. So this your jaw insideyou, here is the tooth. You may have several roots on your teeth, but it's one continuous tooththat is set down in a socket in your jaw.
And that's what we see when we look at the teeth of reptiles and most vertebrates thathave teeth. But these amphibians that are placed in the subclass Lissamphibia have a -- don't havea continuous tooth, they have a plane of fibrous connective tissue that separates the tooth itselffrom this little bony foundation piece that's down in the jaw.
And the term for this is a "pedicellate tooth." Translation of pedicellate is pedestal. Soyou stop to think about a pedestal or a bust or something like that sitting on a pedestal. Well, thisis a reference to this little structure that's down within the jaw.
So all of these animals have pedicellate teeth. They are the only vertebrates that havepedicellate teeth. So that's certainly a unique feature of the group.
The term "Lissamphibia" is reference to the fact that they have smooth skin. "Liss,"smooth. So these are the smooth-skinned amphibians. And that's an interesting feature that is areference to the fact that they don't have scales in their skin. Remember, fish have scales in theirskin, teleosts have ctenoid scales. But all fish have bony plates that are formed in the skin.
Reptiles have scales in their skin as well. But these guys have smooth skin and, hence,they are called Lissamphibia..
The reason that these guys have smooth skin, and that is that they have lost the scales thattheir Labyrinthodonts ancestors must have had, is that they all exchange some significant amountsof oxygen through the skin. So that's another characteristic of these animals is 02 uptake throughthe skin.
And in order to be able to take oxygen up through the skin, you can't have a lot of bonybarriers between the air and the blood. And that's why they have lost their scales.
They all also have mucus glands in the skin. The mucus glands that secrete theproteinaceous mixture of that keeps the skin moist. And that's also an adaptation throughbreathing through the skin, taking up oxygen through your skin. You are going to have very thincells, making up the layer of the skin and no scales, so that you have a minimal barrier betweenthe blood and the air, and the mucus glands are going to be necessary in order to protect that skinfrom being dried out.
So the smooth skin and the mucus glands and the oxygen uptake are 3 morecharacteristics of this group not related to pedicellate teeth.
And finally, this is the most primitive group of vertebrate animals have to a 3-chamberedheart and a. double circulation with both a systemic and pulmonary circuit.
So these are the characters that unite these animals into a single subclass.
There are 3 orders in a subclass. The first one I'll talk about is the order Urodela. Theseare. salamanders. And there are about 300 species of salamanders in the world.
They're the ones that look the most like the Labyrinthodonts. They are the ones that have4 good limbs, and they have a good tail, good sized tail. And as you will see, those are not thingsthat are found in the other orders.
Salamanders are normally found in tropical regions in the world. Although, they are alsofound in the temperate regions. And they are really the greatest diversity of salamanders is in theNew World tropics. But they are quite a few even in the temperate regions in the New World.
"New World" is one of these sort of terms that the Europeans coined referring to Northand South America as the new world as though it wasn't old as the old world. Obviously, by"tropics" I mean areas that are closer to the equator.
So North America is New World temperate. The New World tropics are down in centralSouth America. There are more species of salamanders in the State of Tennessee than there are inthe entire continent of Eurasia.
So for some reason they have their greatest diversity here in North and South America.But they are really more of them in New World tropics than there in the New World temperateregions.
The characteristics are shown here. They have 4 limbs and a tail. But they have someother things that. are weird. They have no ear drum. And they do have a sense of hearing, butthey way in which they detect vibrations through their front limbs. They have a ligamentous kindof connection between the scapula in the. back of the skull, sort of like a bow string kind ofarrangement, that transmits vibrations. So they feel vibrations more or less.
When somebody is walking along the ground, creating vibrations in the soil, thosevibrations are transmitted through the animals legs up to the scapula. And then from the therethrough this ligamentous structure to the back of the head, where the hearing structures are.located. Really strange arrangement.
Natural history of salamanders, the larvae are -- have very diverse feeding strategies, thatis some of the larval salamanders are insectivores. Some of them are carnivores. Some of them arescavengers. Some of them are even cannibals. Some types of baby salamanders actually feed ontheir siblings. The adults are mostly carnivores and insectivores.
Some of them are pretty respectable carnivores, by the way. That animal shown at thebottom of the page, which is an Amphiuma has a common name of "Congo eel." It's found in theSouthwestern United States. It has a -- you wouldn't even want to mess with one of these guys.They get to be 18 inches, maybe 2 feet long. They have a whole mouth full of little razor-sharpteeth, and they attack rats and win.
And you wouldn't even try to pick one up or you'd get a really nasty bite from a Congoeel. So they are pretty respectable carnivores. And salamanders are friendly. Most of you haveseen one in your life.
Reproduction involves swimming behavior. Sometimes brightly colored patches of skin.You have probably seen salamanders that have pretty red spots or red bellies or blue legs. Andthey display these colored things in a stereotype manner to their mates to determine speciesmembership and gender.
And then the males, once the female is interested and responds to its behavioral courtship,then the courtship usually occurs in water in salamanders, in freshwater, even the really highlyterrestrial salamanders will go back into freshwater ponds in order to breed. And the male swimsalong and secretes a pheromone from little glands on his hind limb that are called hedonic glands.
So the male is secreting these hedonic glands, that's a part of courtship for speciesmembership. The female smells this pheromone in the water, and if she is reproductively ready, ifshe is physiologically ready to mate, then she will follow him.
And he is going to swim along, and then he's going to settle down at the bottom of thepond. And he is going to push out of his cloaca a little gelatinous pedestal that has a little packageof sperm on the top of it, that is called a "spermatophore." This entire structure is .
.. called a spermatophore.
He's going to deposit that on the pond, and then he's going to swim forward until she isover it. And then she is just going to settle down on top of it and pick up that spermatophore withher cloaca and then use the sperm to fertilize her eggs.
And you can see this behavior in the "Life on. Earth" videotape where they are courting.The male starts swimming along, and then he settles to the bottom, and then he swims forward,and as she settles down, and that's what is happening is salamander sex right in front of you on thescreen.
Embryonic development is very diverse among these animals. So, obviously, that'sinternal fertilization, because the sperm are fertilizing the female eggs within the female'sreproductive tract.
Embryonic is very diverse. Some salamanders. are oviparous, usually laying their eggs inwater. Some salamanders are oviparous and lay eggs on land, but not very many. And some ofthem are viviparous. Some give birth to live young after they develop within the female'sreproductive tract.
But what comes out of the egg, or what comes out of mom at birth is a larval form. Andthis is also something that's fairly unusual among vertebrates. Most .
.. vertebrates exhibit direct development. But in the Amphibia, there is going to be a larval formwhich is then going to undergo metamorphosis into an adult form.
So the process is metamorphosis. And the product is an adult form. And one thing that'sreally interesting that is the because, in general, the larvae are aquatic, and many of the adults areterrestrial and live -- except for the breeding season they live their whole lives on land.
Some of the kinds of characteristics which change during metamorphosis are things thatreally relate to just whether you are going to be in an aquatic vertebrate or whether you are goingto be in a terrestrial vertebrate.
In other words, there are some things that you will need if you are going to live in waterthat you don't need if you are going to live on land. And vice versa, there are some things youneed to live on land that you don't need to live in water.
And that's the kind of change that occurs during metamorphosis. Obviously, part of whathappens during metamorphosis in all species is going to be the development of functional gonads.A larval form does not have gonads or testes or ovaries. Those organs are present in amicroscopic structure, but they are not even an adult sized organ. They obviously have todevelop, but .
.. don't have an adult size structure.
So some of the larval structures that change during metamorphosis, because there aresome species of salamanders where the adults are aquatic, some of those structures don't develop.When we look at a whole bunch of species salamanders we some species where these develop,these adult structures development; and we some species where they do not develop because theadult is going to be aquatic.
So, for example, larvae usually have fairly weak. limbs. And that adult duringmetamorphosis can development into somewhat stronger limbs. No salamander is going to beseen as a really buff character. You look at the picture of the top of the page there, you can see, atthe top, an adult tiger salamander and immediately. below it a larval one. And the legs of even anadult terrestrial salamander are not going to big and strong, because these guys don't get to bevery big.
But the fish like-tail changes in the ones that are going to be terrestrial. So they just losethe fish-like tail. That's a reference to the fin-like structures that extend above and below the tail.If look at that picture in the upper left-hand corner, there is a fairly meaty interior portion to thatlarval tail, and there is sort of fin-like structures that extend above it.
.. and below it. But that's what is meant by the fish-like tail.
The larvae have no eyelid. An eyelid is really only something you need if you are aterrestrial animal where the eyes are in danger of drying out. So they have. to develop an eyelid ifthey are going to be a terrestrial animal.
The larvae have a spiracle, which is an opening on the side of the skull which is thattrapped gill slit. And remember we talked about that? It's sort of a gill slit kind of structure. Andthey are going to lose that. The adults won't have a spiracle.
Larvae have external gills, which are also going. to be lost. The external gills, you can seeif you look at the larval in the upper corner, there is little feather-like structures on the back of thehead. They are not the same as a fish's gills. Remember fish lost their gills -- or the rhipidistia gaveup their gills when they. invaded land.
They guys have external gills as a mechanism of getting oxygen out of water.
And in a sort of a classical or sort of stereotypical salamander species where the larvalform is a highly aquatic, exclusively aquatic, and the adult a terrestrial, you will see all of thesechanges taking .
.. place.
But in some species of salamanders, the adults are going be to just as aquatic as thelarvae, so none of these things take place. And in some specie of salamanders the adults are sortof semi-aquatic semi-terrestrial. Some of these changes take place and others do not.
So these characteristics which are part of metamorphosis are something that the differentspecies of salamanders do or don't do part of as it suits the lifestyle of the adult.
And they can reverse some of them, too. Some of the terrestrial ones, when they go backinto the water, they will redevelop that fishy extension on their tail to be able to swim through thewater and court the females.
This retention of some larval characteristics in. an adult is called "paedomorphosis."That's the retention of larval characteristics in a adult organism, and it's a characteristic of thesalamander.
The next order is the one that you are probably the most family with. And this is the orderAnura. These are the frogs and toads. This is the largest order of the amphibians. There are about2000 species of anurans.
They are also all found in tropical and temperate regions of the world. And there areactually something .
.. like 7 or 8 families among these 2000. These 2000 species are divided into a number ofdifferent families.
And one of the families is called the true toads and another family is the true fogs, butthere are tree frogs and spade-foot toads. And those words "frog" and "toad" don't have a reallygood consistent taxonomic significance when we're talking about these different families.
Basically, a frog is a highly aquatic anuran and is going to have a fairly smooth skin toreduce drag as its swimming through the water. It's going to have really big hind limbs forjumping. And it's going to go very a good swimmer.
And a toad is going to be a more terrestrial type of anuran. It's spending a larger portionof its life in land, and so it's going to have a dry kind of warty skin. It's going to be kind of bumpyand rough, because they don't need to be smooth for streamlining. And they cannot afford to havereally tiny front limbs, because when these guys jumps they are going to land. They have a littlebit bigger front limb to break the fall after a jump.
So they are these tendencies for any family that's called a frog family to be more aquatic,smooth skin, and larger hind limbs. And any family that toad as part of its name is going to moreterrestrial, dryer skin, wartier .
.. looking kind of critter with somewhat larger front legs.
So you have pictures here that show some of these things.
They are probably among the most highly specialized of the vertebrates in terms of theirmode of locomotion. They exhibit what is called saltatorial. locomotion. And that just meanshopping.
So they have a tremendously reduced number of reduced vertebrae. They have a reallybizarre pelvic girdle. They have tremendously elongated hind limbs, a whole series of specializedadaptations to make them into really, really good jumpers.
Feeding among anurans, the larvae are herbivores. A larval frog, as you possibly know, iscalled a. tadpole. And these guys are herbivores feeding on plant material.
Adult anurans are usually insectivores. But there are also some of them that are prettyrespectable. carnivores. And there are actually species of frogs that get to be -- there is one downin Argentina, Argentinian honored toad, which is almost about a -- one of the big forms are, like,about 9 inches in diameter. They look like a big cow pie. They will attack rodents.
I mean, you can imagine trying to grab a wild mouse or rat, you would be in danger ofgetting seriously bit. And these frogs be able to take them on. But the majority of anurans areinsectivores and are catching various kinds of flying insects by using their tongue.
They are very unusual in that the tongue is attached to the front of the mouth and flapsout. So the back end becomes loose and sticky -- you have probably seen footage of frogscatching flies.
There are also among the more vocal of all of the vertebra animals, particularly inassociation with their breeding behavior.
Normally they are going to return freshwater ponds in order to breed, usually in thespringtime. And it's not usual for there to be several species of frogs all breeding in the samepond. So each species has to have ways of setting up territories. So they produce basically 3different kinds of vocalization, 3 different kinds of sounds.
They have a territorial call, which is a signal that goes from a male frog to another malefrog of the same species that says get out of my territory.
And then they there is an advertisement call, which is the signal that goes from male frogto all the other female frogs of the same species, and it's very specie specific. It has a particularpattern, particular frequencies. Because the males will all congregate in these freshwater ponds,and then they make advertisement calls -- after they set up their territory they make theseadvertisement calls to advertise to the female frogs that, you know, this is where the party istonight.
And so they make these sounds. And it's been demonstrated by neurophysiologists thatthe sensory apparatus of the females, the eardrum and the inner ear structures that are responsiveto sound are tuned to the frequencies of the males calls for each species.
So the females have particularly sensitive hearing for this frequencies of sounds that areproduced by males of their species, and they're all very different. When you see the "Life onEarth" videotape, you'll hear this great cacophony of sounds produced by these frogs.
Now, once the females come to the pond where the males are, then they are going tomate. And that. process is called amplexus. The male climbs up on back of the female, and hegrabs her around the body, sort of behind her limbs. He has these really weird muscles that areattached to his sternum and have a big long tendon that goes all the way down to his wrist.
Those muscles bulk up in the springtime. And they can grab on and squeeze her and helpsher push the eggs out. And then he fertilizes those eggs by depositing his .
.. sperm in the water.
And so as soon as the female comes into the pond the male jumps up on the back of herand attempts to perform amplexus. The problem is that frogs have very simple world view. Andit's not surprising because they have a really small brain.
They basically look at life as if you are bigger than me, you are carnivore and I have toget away from you. If you are smaller than me, I'll eat you. When we used dissect frogs in here wewould find baby frogs in the stomachs of the frogs that we were dissecting. And if you are aboutsame size as me, you're a female and let's perform amplexus.
The problem is that in these ponds, where there are many different species, sometimes thefrog hopping through your territory was a member of another species that didn't pay attention toyour territorial call. And so a male frog jumps up on the top of another male frog and he startstrying to perform amplexus.
And so the third type of call that frogs produce is to extricate themselves from thisrelationship that doesn't have any future. It's called a "release call." And just basically means, "Getoff my back. There is no future in this relationship." .
They produce 3 different kinds of calls: The .
.. territorial calls to tell other males to stay away from their territory. The advertisement calls toattract the females. And the release calls to terminate amplexus.
So it's the release call that is given by the guy on the bottom that says you have thingsconfused here.
And you can actually -- we'll have some live frogs and lizards in the lab. If you grab afrog, a male frog right behind the legs and squeeze him a little bit, he'll make this sound -- which isa release call.
And that's the same in all species. That's the thing that's neat is these territorial calls arespecies specific, the advertisement calls are species specific, but the release call is the same in allof them.
Now, development is usually oviparous. Although, it's interesting to note that there aresome species that have internal fertilization. There are couple of species of frogs in which themales has an intromittant organ and fertilization is internal, and they are viviparous. They developinside the female's reproductive tract.
That generalization is of interest to me because it turns out that most of the -- almostevery major group of vertebrates has evolved viviparity, with the only real exception to that arethe birds.
So there are some amphibians that have internal fertilization and are viviparous. Most ofthem are .
.. oviparous. There is a huge range of different kinds of adaptations that have evolved to protectthe eggs.
Some of these are presented along the bottom of the page. They make foam nests. In thelower left-hand corner, that looks like a big flower next to those frogs performing amplexus, butthat's actually a foam nest that they have made.
Sometimes they deposit them up in the leaves to get them away from predators.Sometimes either the female or the male will carry the eggs around to protect them. And there aresome really bizarre examples of where the eggs actually sink into little pockets on the back of thefemale. And then development occurs in these little pouches underneath the skin of the female.
And then there is also this neat footage in the "Life on Earth" videotape on Darwin's frogin which the male takes the eggs and swallows them down -- not into the stomach but down intothe vocal sacs that are located beneath his jaw. And the young tadpoles develop in there, andmetamorphose into small frogs, and what pops out of dad's mouth is a little tiny frog.
They have gone to some really amazing lengths, evolutionarily, to protect the eggs.
Now, the last order is the order Apoda.
Anura, by the way, means without a tail. And .
.. that's a reference to the fact that frogs undergo a very dramatic metamorphosis, a much biggerand more extensive metamorphosis than occurs in the salamanders.
We don't see adult frogs that have tails. A tadpole does not have arms or legs; it has a bigtail. Adult frogs grow legs and lose the tail. So metamorphosis. is much more extensive. AndAnura is a reference to the fact that in the adults you have no tail.
The other order is the order "Apoda," which means without legs. This is a reference tothe fact that the members of this order, there are about 160 species, they're called "Caecilians" or"apodans," as they are referred to, don't have any legs at all.
They are burrowing. That is, the technical term for a burrowing vertebrate is that they arefossorial. They burrow through the soil. And what we find among fossorial vertebrates I havealready described when I was talking about the hag fish, they have vestigial eyes.
And these guys have no tail at all. They have a terminal anus, that is the end of thedigestive tract is at the back end of the body, sort of like a worm.
In fact, these guys, if you saw a little one you might originally -- your quick immediateimpression might be that you are looking at a worm. But if you look closer, you will see that that'snot true, because they .
.. have an obvious jaw, and some of them get to be truly big.
The largest one is from the Southeast Asia and it gets to be about an inch diameter andfour and half feet long. They can get to be pretty good sized animals. They burrow through moistsoil and feed on worms and small invertebrates. They have internal fertilization.
And some of the species as is illustrated here, some of the species are oviparous and carefor the eggs. So that's a female Caecilians wrapped around her egg mass in the middle of the pagethere.
And some of them are viviparous. The animal that's shown at the bottom there is a larvalmember of the genus Typhlonectes. And Typhlonectes, is one of these viviparous species. And sothe larvae are going to be developing within the female's reproductive tract.
And Typhlonectes to me is a really fascinating animal. I don't think there is any animalthat has ever been studied that gets as pregnant as these guys do.
A female Typhlonectes might be 50 centimeters long, a foot and a half long. And she'llhave 9 babies, each of them 20 centimeters long. So that just before birth, if you weigh the mom,and then after she gives birth to the babies, and you weigh the babies, and the babies weigh 60percent of what the total weight of the mom, plus babies was. That would be like a 150 poundhuman giving birth to -- and we're not talking about uterus and amniotic fluid, we're talking aboutbabies weighing 60 percent of. the combined weight of the mom and the baby. That's a reallyamazing pregnancy that they have.
It's supported by uterine milk. They don't have a placenta. But they secrete fluids into theuterine fluid, and the babies ingest that. So nutrition is in the form of uterine milk.
So those are the Caecilians. They are fossorial, tropical animals found in the tropicalregions of the world .............
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ZOO 138 Lecture Transcripts (W'97) Page 7