3840 Chapters 23 Chordata

welcome back everyone in this lecture we're covering phylum chordata this is the last phylum that we'll cover this semester in our next lecture we'll cover subphylum vertebrata but this is the last official phylum that we're going to be covering and this is chapter 23 in your textbook phylum chordata contains three subphylums we have urochordata which include our tunicates and those are shown here we have cephalochordata which include our lancelets which are these guys here and then we finally have vertebrata which are the vast majority of animals you probably think of when you think of an animal and this includes amphibians fish reptiles both avian and non-avian and mammals so all of the diverse animals that are shown in all of these different groupings are vertebrates so all chordates share some key characteristics and i've listed them here but those that are really important to remember are these five characteristics that i've put in green in this in this list and these characteristics are unique to chordates only chordates have these so one are pharyngeal gill pouches or slits and these are also present in group of organisms we didn't talk about called hemichordates but it's considered to be very unique characteristic to chordates and then we also have dorsal hollow nerve cord notochord endostyle or thyroid gland and post anal tail and so over the next couple slides we're going to talk about each of these characteristics in more detail to be considered chordate an animal has to have all five of these characteristics at some point in their life cycle so they don't necessarily have to have all these characteristics as an adult they can have them as larva and then lose them as an adult but they have to have these five characteristics at some point in their life one of the first structures that develops in chordates is the notochord and the notochord is cartilaginous cord that lies on the dorsal side of the animal and this cord is ventral to what will develop into the central nervous system so they're showing you that in this picture here this highlighted purple region is the notochord and then this white region here that's not quite filled in yet would later become the central nervous system and we'll talk about what that white structure is in second and so the notochord is composed of couple of different layers at the very center of the notochord it's fluid filled and it's not necessarily just like fluid-filled cavity like what we saw in nematodes instead it's whole bunch of cells that have large vacuoles that are filled with fluid and then there's also lots of fluid that is filling up all the spaces between the cells so that's what mean by it's fluid filled it's composed of cells but those cells have lots of fluid in them and around them and then on the outside of that there's fibrous sheath and then outside of that there is an elastic sheath so it's almost like tube within tube within tube and this nodochord acts as hydrostatic organ for these animals and so it's very similar to what we talked about in nematodes and in analytes as hydrostatic skeleton for movement and support but it's not exactly the same because it's not just fluid filled cavity it's composed of cells and the notochord allows for the body to have some more support like we just talked about it stiffens the body up so you're not kind of like loose bag of fluid and then it also allows for more secure attachment for muscles and then we invertebrates we like say we because we are humans are vertebrates we later replace this notochord with vertebrae and these vertebrae can be made of either bone or cartilage and they're mesodermally derived and so what will happen is that that notochord will be replaced by several vertebrae that look very similar to this the dorsal hollow nerve cord is tube derived from ectodermal tissue that runs along the dorsal side of the animal and it's also dorsal to the nodochord so on the last slide mentioned that the notochord was ventral to structure that later gives rise to the central nervous system that's what we're talking about here so in this image you can see that the dorsal nerve cord is highlighted in blue and it's dorsal to the notochord which is highlighted in purple and in all chordates the nerve cord is hollow at some point in some chordates as they're undergoing development the hollow tube will become filled with cellular material however their all chordates have hollow nerve cord at some point in their development and we can see why when we look at the development of the dorsal hollow nerve cord so we begin with our early animal and early in development there's piece of ectodermal tissue that's on the dorsal side and this tissue is referred to as the neural plate and the neural plate is starts out relatively flat but then it starts to fold in on itself and as it's folding in on itself two regions will form called the neural folds so you have one here and one here so as the neural plate continues to fold in on itself it'll bring the neural folds closer and closer together and then once the neural folds meet they will pinch off from the rest of the neural plate and they'll also separate from the tissue that's lateral to it and so this will give us three structures it will give us the epidermis which is composed of the tissue that lied lateral to the neural plate so that those pieces just meet up and and connect and then we have our hollow nerve cord and then we finally have another structure called the neural crest and the neural crest will later branch out to form peripheral nerves the all vertebrates when it comes to the hollow nerve cord have couple of different structures one we have skull that encases the brain and the skull can be made of cartilage or bone but the skull helps to protect the brain from any potential damage also the brain will be tripartite so it's composed of forebrain midbrain and hindbrain and then we also have our vertebrae right so the nerve cord will actually run through these various vertebrae through the vertebral vertebral arch which is also known as the neural arch pharyngeal pouches of pharyngeal slits occur near the pharynx and they're result of invagination of ectodermal tissue and evagination of endodermal tissue near the pharyngeal cavity so it looks something kind of like this we have our ectodermal tissue and it will invaginate and then we have our endodermal tissue that will evaginate and that gives you these little pockets or clefs if these pockets meet up and cause break through the cavity then this is results in pharyngeal slits but if those pout those pockets don't meet up and they stay separated and don't break through the cavity then they're pharyngeal pouches and so you can see an example as well in this top picture they're showing you some pharyngeal pouches they show how that endoderm and ectoderm don't quite meet up so you get pouches and then you can also see them in kind of real real life picture in this picture of fetus their pharyngeal pouches or pharyngeal slits are there and then same with an up close picture here and then also in this image here adding to this kind of core image that's been adding the various key characteristics of chordates to it they've added the pharyngeal slits which are highlighted in that kind of yellowy color so these pharyngeal pouches and slits first evolved as filter feeding structures their these see slits and pouches are coated in mucus when they're used for filter feeding and this mucus is derived from structure called the endostyle which we'll talk about next but they were primarily used for catching small bacteria and plankton in the water which was then trapped in the mucous and then able to be digested by the digestive system however in more advanced chordates like fish and sharks both bony and non-boating fish these structures have developed into gills and they play role not necessarily in feeding but in respiration in combination with that we see aortic arches in the pharyngeal cavity mean pharyngeal pouches and slits and they play role in carrying blood to the pharynx but in more advanced vertebrates and chordates we see that these aortic arches play role in gas exchange especially when we're talking about gills and the pharyngeal pouches and slits develop into several different structures and vertebrates including the jaws and we talked about in gills and they also develop into variety of other structures that i've listed here the structure responsible for producing the mucus that lines the pharyngeal slits and pouches is called the endostyle and all chordates have an endostyle or thyroid gland and it's and or because the endostyle and the thyroid gland can perform some similar characteristics when it comes to iodide protein secretions or compound secretions and so the endostyle in addition to producing mucus has also been found to secrete iodinated proteins and then if you know much about the thyroid gland which is shown in this picture here the thyroid gland is responsible for also secret secreting iodinated hormones so the endostyle is thought to maybe have been well thought to be homologous or have been an early version of what would later do be developed into the thyroid gland in more complex organisms like vertebrates the final key characteristic of chordates is the presence of post anal tail and the post anal tail is muscular projection that extends past the anus and it's primarily responsible for locomotion and this is why it's muscular right so it has many segmented muscles called myomeres that will anchor themselves two parts of the tail and the notochord and this allows for the tail to move generally in an undulating or wagging type motion and it was primarily evolved so that our fish-like ancestors were able to properly move through the water column now in some animals like us we have lost our the vast majority of our post-anal tail and we only have very very small remnant of it and it can be seen here as your tailbone right so your coccyx or your tailbone is the only thing that we have left of post anal tail but during development as fetus you can clearly see that post anal tail is present in all chordates including us now that we've talked about the key characteristics of chordates let's talk about the different subphylums we'll begin with one of my personal favorites urochordata and euro eurocodata include the tunicates tunicates are these small aquatic almost alien looking animals but they're actually kind of cute and you'll see some pictures in second but they get their name from the cellulose tunic that surrounds their body so that can be seen in this picture here and when you look at an adult tunicate you would not think that it was chordate and this is because in the adult form they're missing lot of the key characteristics we just talked about however they are chordates because all of those key characteristics of chordates are present in the larval form it's just as the larva is undergoing metamorphosis into the adult some of those characteristics or those structures are reduced or or lost so we can see that if we look at the diagram below you can see that in this larva they have the nerve cord tail notochord pharyngeal slits and you can't see the endostyle here because of the way that the diagram is drawn but the endo style is present as well but as you follow along with this diagram you'll notice that some of these structures become smaller and smaller and then some of these structures are ultimately lost so two structures that are lost as the larvae undergo metamorphosis include the notochord and the tail the notochord is highlighted in purple and you can see how it gets smaller and smaller until in the adult form is completely absent same thing with the tail that projection gets smaller and smaller until it's completely lost and then some structures are just reduced we can see that with the nerve cord which is highlighted in blue the nerve cord becomes smaller and smaller as well but it doesn't completely disappear instead we end up with single ganglia that is composed of this tissue and so that's why that we we know that these these tunicates are indeed chordates because all those key structures are present in the larval form even though they're lost some of them are lost in the adult form tunicates can be either sessile or free living and there are couple of examples that are given here the top two up here are obviously session they're attached to substrate and then the ones down here are free living they can also be solitary so each of these tunicates can live by themselves they can be colonial and some examples include here and here both of these are colonial or they can be compound and this is an example of compound tunicate and then when you have compound tunicate they basically share an x-current siphon or they'll share resources if you will so this tunicate here that's compound tunicate looks like it has whole bunch of dots on it and that's because each of those holes is an incurrent siphon for an individual tunicate and they all share an x-current siphon which is this larger hole that's located here so looking at the siphons that leads us to talking about the feeding mechanism so tunicates have an anterior and dorsal siphons and the anterior one is on their anterior region and then their x-current siphon which is on their dorsal region and they use these for filter feeding so how this works is that food is brought into the in current siphon and it'll be filtered through the pharyngeal slits here so you can see the pharyngeal slits in the tunicate are almost like meshwork and the endostyle which is like really really small in this picture it's little bit hard to see but the endo style which i'm going to highlight in red here we go produces mucus that will coat those pharyngeal slits and that mucus helps to trap any platon plankton or bacteria that are in the water that's being brought in that water then travels out of the pharynx to the atrium which is here and then from the atrium it will travel out of the current siphon on the dorsal side the food on the other hand will travel from the mucus in the endostyle down to the stomach and the intestines and any waste will be removed out of the anus and then the anus is located near the x-current siphon so this will allow for any waste to be removed from the body safely out of the x-current siphon and all tunicates are mono sorry monoisis they can also undergo either sexual or asexual reproduction and their fertilization is external so they will release their gametes into the water column via their gonopor which also is located near the atrium and the x-current siphon and though so they'll release their gametes into the atrium they'll travel out of the atrium to the through the x-current siphon into the environment and be fertilized by either their own sperm or eggs or the some someone from nearby tunicate some examples include sea squirts and salts and wanted to show you guys this is an example of tuna kit they're quite interesting so we just talked about how they have pharyngeal slits that are covered in mucus that allow them to to feed well there's these salps that have kind of like weird honeycomb type shaped house like dome shaped house and it's made of mucus and they use this house to catch plankton in the water and then they will ingest that mucous kind of ball that they've made for themselves and that will allow them to consume any plankton or bacteria that got attached to that mucus and then once that ball is gone they'll then go on and make another one and it only takes them couple minutes to make this this structure so this allows them to more efficiently get the food they need especially with them being as small as they are so this is the actual tunicate that i've that this is the actual tunicate and then this is the what they call the house which is responsible for catching the food so water comes in and gets stuck to the mucus that's located in here and then all the extra water goes out another subphylum of chordates is cephalochordata and cephalochordata include the lancelets as you're going through your textbook you'll see different term other than lancelets you'll see amphioxus it's an older term but it's far more encompassing so you'll see that just be advised this is what they're talking about and all the cephalochordates have very clear chordate characteristics in their adult form so if you look at this image on the bottom here you can see all of those chordate features that we talked about are present in the adult form including myomers that are attached to the nodochord and these myomers allow for the animal to be able to swim and to burrow they mostly like to be burrowed in some sort of substrate however they are modal so they can get from place to place using these myomers to push the the tail and undulate it so they can swim from place to place they are aquatic and they're filter feeders and they filter feed by bringing in water from their mouths into the pharyngeal slits and the pharyngeal slits are coated in mucus that comes from the endostyle from there the water moves to the atrium which is kind of the space between the pharyngeal slits and the the epidermis of the animal and from the atrium any excess water will be removed via the atrial pore the mucus is removed from the gill slits and gets moved into the digestive tract for any bacteria or plankton to be digested and there are many many features that lancelets have that are in common with vertebrates but they are missing one key key feature they do not have head with specialized sense organs and so this is why they're not included with the vertebrates even though they have lot of similarities to vertebrates and so at one point it was thought that they were vertebrates until we noticed that all other vertebrates have specialized sense organs on the head and they do not have them and so it's been hypothesized due to the similarities that the cephalochordates are likely the sister group to vertebrates the cephalochordates have closed circulatory system but they do not have heart instead they rely on peristaltic movement of blood through the vessels to get from place to place so they start with ventral aorta and then blood moves through the vessels from the ventral aorta past the aortic arches and then to dorsal aorta and then from the dorsal aorta the the blood will move through the capillaries and various tissues and then return back to the ventral aorta via vein this whole process is not used to transport gases throughout the body no gas exchange occurs in these animals in that way they use diffusion for gas exchange instead what they use this circulatory system for is to transport nutrients throughout the body rather than to transport then to transport gases lancelets and all cephalochordates are dioecious and they also undergo external fertilization so they release their gametes into the water column where fertilization occurs and then their larva will quickly develop from an egg into small larva and then they will develop into full-grown adult lastly we have subphylum vertebrata and this includes all of our vertebrates and all vertebrates have some key share characteristics that are not present in subphylum zero chordata or cephalochordata number one of course the presence of vertebrae and the vertebrae have replaced the notochord in invertebrates and so you can see the vertebrae very clearly in this image here all right and the these hardened vertebrae allow for better attachment points for muscles so this contributes to them being more active animals than the cephalochordates or the urochordates they also have bony or cartilaginous endoskeleton this provides more structure for the animal but also allows them to grow larger in size than if they had an exoskeleton so we talked about in arthropods with an exoskeleton you can only grow so big before you have to shed it right but in in the benefits of having an endoskeleton is that you can grow in size as long as your skeleton is intact you're good to go and this also provides strong anchoring point for muscles which further contributes to the more active lifestyle and this endoskeleton was first thought to have evolved as method of mineral storage so the calcium that you have in your bones in your body plays lot of major roles especially when it comes to your nervous system and how your cells function so it's thought that the bones and cartilage were first derived in order to store minerals and then later became useful for muscle attachments and for providing structure and support then all vertebrates have cranium so they have skull they have muscular pharynx and muscular digestive tract so unlike what we saw in urochordata and cephalochordata in both of those cases didn't mention this before but when they're when they're filter feeding this method is primarily facilitated by cilia some urochordates can use muscle contractions to bring in water past their pharynx but for the most part they use cilia but in chordae mean in vertebrates we use muscles and then also for our digestive tract in both the euro chordata and cephalochordata their food is moved to the digestive digestive tract via cilia however in vertebrates we move food through our digestive tract using muscle contractions we also have various digestive glands that are not present in other subphylums including our liver and pancreas and then additionally with that we also have gas exchange that occurs in the pharyngeal arches so in the pharyngeal gill slits so the skill slits in vertebrates is not strictly just for feeding it also plays major role in gas exchange and respiration and then vertebrates also have chambered heart and red blood cells which we see hearts in both urochordata and cephalochordata but or aortas in the cephalochordata but they don't have this complex chambered heart that helps circulate nutrients as well as gases using the red blood cells and then we have kidneys as well and then one of the biggest things when we're looking at vertebrates compared to urochordates and cephalochordates is the sensory organs so encephalochordates and urochordates they don't really have well-developed brains but vertebrates we do we have tripartite brain with the four gut of forebrain midbrain and hindbrain and we also have specialized sense organs that are paired on the head so of course we have two eyes two ears right and one one knows the two nostrils right so we can sense the environment around us and these these structures were actually developed from new type of embryonic tissue so we have two new types of tissue that are present in vertebrates that are unique to this subphylum that includes the neural crest which later develops into the cranium pharynx parts of the teeth and parts of your endocrine endocrine glands and then we also have ectodermal placodes which contribute to the development of your nose eyes ears brain if you are fish your lateral line and electro receptors there are many questions that remain unanswered regarding the history and evolution of vertebrates however we do have couple of key clues that give us some information on the evolution of vertebrates and how we ended up where we are and what we are today and so one kind of big clue that we have is from an actually extant species called lamprey and lamprey is jawless fish and the larvae of lamprey are thought to have body plan that very closely resembles that of the common shared ancestor of all vertebrates so you can see that picture in this diagram here and if you look at this diagram one of the first things you might think is that it looks very similar to the diagram we just looked at with the lancelet but unlike the lancelet the lamprey is an actual vertebrate and this is because the lamprey larva even in their larva stage have key characteristics that all vertebrates share many of these characteristics are not present in the lancelets which is why they are more than likely sister group to all vertebrates and these key characteristics of vertebrates that are present in the lamprey larva include chambered heart tripartite brain paired specialized sense organs pituitary gland kidneys liver gallbladder and pancreatic tissue so all of these structures help to differentiate the lancelet from the lamprey larva even though they do look very similar and have lot of similar structures additionally the gills in the lamprey are not only used for feeding they're also used for respiration this is also another key difference and you can see that we're starting to get closer and closer to understanding the shared common ancestor of all vertebrates that they likely had evolved their gills from just being feeding structures to being feeding and respiratory structures we also know that vertebrates began as jawless fish and not only because of the lamprey larvae but there's also other fossilized evidence that gives us some clues that jawless fish fir came as one of the main stages in the evolution of vertebrates so the the group that jaw the ancient jawless fish belong to is called agnatha and agnathans include all jawless fish so this is paraphyletic group because it's not just the ancient ones but also modern day jawless fish and they all these ancient jawless fish were covered in armor and this helped to protect them from predators so this is heavy good advantage as well they also had fins but their fins were unpaired this allowed them to move through the water column to capture food but they weren't really all that coordinated so it's unlikely that they were hunters until active predators until they developed their paired of fins so we began with unpair fins and they were likely filter feeders and then over time as the fins became paired they likely became active predators when looking at vertebrate evolution the development of jaws completely changed the game this allowed vertebrates to have access to food sources that they previously did not have access to and changed many of their habits which has allowed them to be as successful as they are today and so when we're looking at jawed vertebrates not all vertebrates have jaws of course we have lampreys and hagfish that are extent today that are jawless but the vast majority of vertebrates do have jaws and all vertebrates that have jaws belong to group called natastones this is monophyletic group and it includes all extinct and extant jawed fish and tetrapods so it's lot of diversity and these jaws allowed for the success of the organisms because like mentioned they had access to food that they didn't have access to before and they also were able to have behaviors that allowed them to occupy new niches that they couldn't before so for example let's look back at the history of these fish so we started with jawless fish with unpaired fins and so that means that they were probably swimming in more haphazard way through the water column and with their mouths open hoping to catch bacteria and plankton on their gills for feeding and then from there we saw the development of paired fins and those paraffins allowed for more coordination so those fish were likely active predators and because they still didn't have jaws they were probably still feeding on invertebrates and other soft foods however once we see the development of jaws in our jawed fish that allow for them to eat not only invertebrates but also to be able to eat other vertebrates as well and then the jaws are actually developed from modified gilt arches so there's lot of evidence to suggest that the most anterior gilt arches eventually kind of came to be the jaws that we know today and then eventually we have the the teeth as an addition and then i'm as mentioned with the jawless fish they developed their paired fins we still see those paired fins in our jawed fish which allows them to be lot more stable in the water column so they can be more efficient and active predators then we also see some paired limbs which eventually give rise to our terrestrial tetrapods this concludes our lecture on phylum chordata you can go ahead and read chapter 23 and do the chapter 23 connect assignment i've also included three videos here to help with understanding the the concepts we talked about in this lecture highly recommend that everybody watch the crash course video on phylum chordata because number one they cover all the concepts that we talked about in this lecture but they also give you nice introduction to everything that we're going to talk about in our next lecture which is talking about subphylum vertebrata the only thing that they don't cover in this crash course video is the endostyle or thyroid gland as shared characteristic of chordates and that's because this is recent addition and think that video is little bit older and then also there's an animation here for chordates and then the last video it's just video of larvacean tunicate so that misspoke before and call this tuna get salt but it's larvacean tunicate it's the one that builds house and so it's just cute little video that shows you some live footage of what these tunicates look like so that one is not necessarily so much required but it's just really cool to see what they actually look like in the water column but really really recommend that everybody watch the crash course video on phylum chordata it's only about 12 minutes long so it's not super extensive and think it will really help cement the concepts that we talked about in this lecture as well as give you nice start to what we'll talk about in our final lecture of the semester talking about subphylum vertibrato all right with that said i'll see you in our last lecture and i'll see you next time