Understanding Phylum Chordata: Key Features and Classes

this is the vertebrates lecture part one  of two finally we get to our last phylum   this is phylum Chordata and this means it has a  spine and so Chordata have organ system level organization triploblastic so all three germ  layers one-way guts so an alimentary canal 

going from mouth to anus bilateral symmetry  they're coelomic deuterostomes so mouth forms second segmented and there are four special  characteristics that all Chordata have or had   at least an early development and so that is  a dorsal nerve cord and so nerve cord running  

along the back a notochord and a notochord is  a cartilaginous skeletal rod that supports the   body pharyngeal gill slits and a post-anal tail  so tail after the anus as you can imagine phylum   Chordata is quite large and so we're going to  divide this into multiple different subphylum  

starting with subphylum Tunicata Tunicata are  the tunicates and these are the sea squirts and   so the larvae the early developmental form  has all four other special characteristics   and so your dorsal nerve cord the notochord  the pharyngeal gill slits and the post-anal  

tail and the larvae are free swimming so swimming  around in the water but they're non-feeding once   the larvae metamorphosize into the adult the  only thing left is the pharyngeal gill slits   it now becomes a sessile marine filter feeder  next we have subphylum cephalochordata

cephalochordata translates to head cord and these are  the lancelets or amphioxus and again we could   see all four special characteristics so dorsal  nerve cord notochord pharyngeal gill slits and   the post-anal tail now we get to subphylum  vertebrata and in vertevrata the nodal cord  

is the backbone and we're gonna divide it into  multiple different classes starting with class   Agnatha the prefix a- means without so do not have  and so class Agnatha do not have jaws and so their   mouth is just an opening they cannot bite down  so no jaws and this includes the lamprey this is  

the lamprey and the hagfish the interesting thing about  hagfish is they have slime glands and if they are   being hunted by a predator they can secrete slime  to deter predators from trying to bite down and   so they can wiggle their way out from the jaws of  another predator next we have class chondrichthyes  

chondrichthyes translates to cartilage fish and  these are the sharks rays and chimeras and so   for chondrichthyes their skeleton is made of the  flexible cartilage they have external gill slits   so there's no covering covering the gills a  heterocercal tail where the top and bottom  

fin of the tail are different sizes they have  a two chambered heart so just one atrium and   one ventricle they are ectothermic so they're body  temperature is whatever the water is and they have   placoid scales and these are the dermal denticles  and this is homologous with teeth this is why shark  

skin is so rough next we have class osteichthyes  osteichthyes translates to bony fish where their   skeleton is made of bone and so bone is calcified  so it's a lot harder their gills are also covered   and so it's covered with the operculum so there's  a covering here the operculum and the gills  

are underneath they have a homocercal tail where  the top and bottom fin of the tail are the same   size they still have a two chamber heart so one  atrium one ventricle and they're ectothermic and   here they have leptoid scales an so the scales overlap to reduce drag allow them swim  

fast through the water we can divide up Class osteichthyes into two different groups starting with the   ray-finned fish for ray-finned fish the fins have  hard supports and within ray-finned fish there are   multiple different orders and so we have order  Salmoniformes Salmoniformes are the salmon  

trout and char and then we have order Perciformes perciformes include perch tuna bass sheephead   sailfish and Marlin and then we have order  Scorpaeniformes Scorpaeniformes include   scorpionfish lionfish sculpin and rockfish and  these tend to have venom in the tips of their  

rays so you do not want to poke them and then  we have order Cypriniformes Cypriniformes  include carp and goldfish and koi are type of carp  and minnow continuing on with Class osteichthyes and the ray-finned fish we have order Siluriformes Siluriformes are the catfish and then  

order Anguilliformes Anguilliformes are the eels and then we have order Pleuronectiformes  Pleuronectiformes include flounder  halibut and sole these are all the flatfish and   when the fish are young their eyes are on either  side the head and as I get older both eyes move  

to the same side allowing the fish to lie flat  on the bottom of sea floor away from predators   and then we have order Sygnathiformes Sygnathiformes are the pipefish and the   seahorse and this one here is a pipefish and then  riding on top of the pipefish is the seahorse

so for ray-finned fish the fins are supported  with hard fin rays so bones in the fins making   them hard but for the next group these are the  lobe-finned fish and the lobe-finned fish have   fleshy muscular fins and so giving them a more  rounded lobe-like appearance for the lobe-finned fish 

we have order coelacanthiformes coelacanthiformes are the coelacanth and these are a very   ancient fish dating back to the Devonian and  Carboniferous period and the living relatives   of this ancient lineage have not changed very  much in all this time and so therefore they're

also considered to be living fossils and then we  have order Lepidosireniformes Lepidosireniformes are the lung fish and these fish can  breathe air and so they live in areas that have   droughts periodically and as the lake dries up  they can actually hop out of the water and hop to  

another lake and then jump in sometime in the  late devonian one of the species of lobe-finned   fish use their fleshy muscular lobe-fins to  walk on land and then evolved into the   early amphibians and so amphibians belong  to class amphibia amphibians live in the  

water and they can live on land but they still  have to stay close to water for their life cycle   and so when they lay eggs the eggs do not have a  protective outer coat so the eggs do not have an   amnion and they would dry out if they were on  land and so amphibians lay their eggs in the  

water when the eggs hatch the larvae then come  out and so they have an aquatic larval stage   so imagine a tadpole and even adults have to be  near the water and have to keep their skin moist   because they can respire through their skin so  they have lungs but they can also breathe through  

their skin as well they have a three-chambered  heart so two atria and one ventricle and they   are ectothermic so whatever that environmental  temperature is is the temperature of their bodies   for amphibian diversity we have order Gymnophiona Gymnophiona translates to naked snake and these  

are the limbless Caecilians Caecilians used to be  called order Apoda so you might still see that   in some places and these look like snakes or  eels or worms but they are in fact amphibians   and then we have order Urodela Urodela are  the salamanders and mythologically salamanders  

are associated with fire which is weird because  they're amphibians and so they live in water and   so what happened was in ancient times people  would throw logs onto the fire to keep them   warm and then all of the critters will scurry out  including the salamanders and so they came up with the  

idea that salamanders were born from fire and then  we have order Anura Anura are frogs and toads   Anura translates to no tail because as the tadpoles  metamorphosize into frogs the tail disappears and   so now we get to class reptilia reptilia are the  reptiles and these live primarily on land and so  

as the organisms moved further inland then there  was a danger of their eggs drying out and so they   had an adaptation to allow them survive on land and  this is the amniotic egg and so multiple extra   layers of protection to keep the egg from drying  also they have now gotten rid of the larval stage  

so no need for an aquatic larval stage and when  the egg hatches it's instantly simply a miniature   version of the reptile and the respiration is  fully by lungs and they have a three chambered   heart so two atria one ventricle except for  order crocodilia so crocodiles and alligators  

have a four chambered heart but there's still  ectothermic so their body temperature is that   of the environment and to protect them from the  environment they have keratinized scales and so   hardened scales and in some cases a scute scute means shield these are bony external plates  

and so as in the turtle shell for example or the  crocodile skin so for reptile diversity we have   order Testudines Testudines are turtles  and tortoises in the English language typically   turtles are aquatic whereas a tortoise it lives  on land but in other languages they don't make  

that distinction then we have order Squamata Squamata are lizards and snakes and then we   have order Rhynchocephalia Rhynchocephalia is the  tuatara and there's only one genus left of tuatara   and this is Sphenodon and so it looks like a  lizard but it's not at its own order in Rhynchocephalia

then we have order crocodilia crocodilia  are the crocodiles and alligators you can tell   the difference between crocodile alligator by  looking at the snout crocodiles have more narrow   snout where alligators have much wider snout  but you probably won't be looking that  

close anyway next let's go over several different  extinct reptiles starting with order or Ornithischia Ornithischia are the bird-hipped dinosaurs and so  the hip structure resembles that of modern birds   although interesting enough this is not where  modern birds evolved from and so for order

Ornithischia we have the Triceratops because it has  three horns we have the Stegosaurus which has bony   plate all on the back we have the ankylosaurus  which has a club for a tail and Hadrosaurus which   are the duck-billed dinosaurs then we have order  Saurischia Saurischia are the lizard-hipped dinosaurs  

and so the hip structure resembles that of modern  lizards and there are two groups in order Saurischia there are the theropods theropods are carnivores  so things like velociraptor and tyrannosaurus   and then we have the sauropods sauropods are  herbivores and they tend have really long necks  

so a Brachiosaurus and Apatosaurus are sauropods  and then it turns out birds evolved from theropod   dinosaurs so modern birds are like velociraptors  running around that can fly next we have other   ancient extinct reptiles however these are not  grouped in with dinosaurs they lived at the same  

time but they themselves are not dinosaurs and so  we have order Pterosauria Pterosauria means winged   lizard and these are the flying reptiles like  the pterodactyls so Pterodactylus and Pteranodon   then we have order Plesiosauria Plesiosauria means near to lizards and these are marine  

reptiles such as plesiosaur and Plesiosaur had  a really long neck and fins allowing to swim so   therefore it was a marine reptile and then  we have order it Ichthyosauria Ichthyosauria means   fish lizard and these are also marine reptiles and  so Ichthyosaurus resembled that of a dolphin