Chapter 34 Chordates

welcome to chapter 34. we're going to be discussing in the next several segments the origin and evolution of vertebrates however we're going to start with chordates so this segment is focused on chordates it's important to note that chordates are not actually vertebrates so the evolution of vertebrates be with the chordates so if we take look at this tree right here we have the chordates and remember chapter 33 discuss the aquino dermata and this is really tree of all representing all deuterostomes now we've already discussed the aquinodermata and now we're going to move into the chordates and the chordates that are not vertebrates include the cephalochordata and the urochordata and it's important to note that one of their key characteristics is the presence of notochord that's present sometime during their development however there are other three other characteristics that make up you know key characteristics that are that make up the chordates chordates and then of course the vertebrates are group of animals that have vertebrae so it's important to note that the chordates do not have vertebrae so if we move on the chordates like mentioned they all have notochord they also have what's called dorsal hollow nerve cord along with couple extra other characteristics though so the phylum chordata consists of those two basal groups which showed you in the tree and and then it also includes the vertebrates however the two basal groups are not vertebrates now this phylum includes animals that are all bilaterally symmetrical they are also salimate so they have true body cavity and they all have segmented bodies the chordates again phylum chordata include the two groups of invertebrates or the basal groups the urochordata and the cephalochordata and then it also includes all of vertebrates but again the urochordata and the cephalochordata are not vertebrates because they have no vertebrae now the four characteristics of chordates or all chordates in are the following so there is the notochord as previously mentioned the dorsal hollow nerve cord the post anal tail and the presence of pharyngeal slits or clefts some of these species have these traits only during embryonic development some of them retain the traits throughout their development until they're they're adults but all chordates share these traits at some point during their development so the notochord is actually long longitudinal rod it's it's kind of flexible rod and it is present between the dorsal hollow nerve cord and the digestive system the notochord provides skeletal support in most vertebrates however more complex skeleton forms which would make up the vertebrae adults retain only remnants of the notochord in the vertebrate group the nerve cord in the embryo develops from the ectoderm and it rolls to form tube called the dorsal hollow nerve cord okay so again this is dorsal to the notochord and that's why the term dorsal is used to describe the hollow nerve cord this eventually develops into the central nervous system which includes the brain and the spinal cord in most chordates the grooves in the pharynx are called the pharyngeal clefts these are going to develop into slits that open up to the outside of the body really forming the gills in some species so those species that are suspension feeding and these are going to be the suspension feeding structures in the invertebrates which includes the process of gas exchange in vertebrates now this does not occur in the tetrapods the tetrapods develop these pharyngeal clefts develop into parts of the ear parts of the neck and parts of the the head the chordates also have posterior you know tail which is called the post anal tail this is often reduced during embryonic development however it is retained in some chordates this contains skeletal elements and muscles and it provides force of propulsion in many of the species that are aquatic so if we take look at this basic tree we can see that the root of course would represent those that are chordates remember the deuter stones also include the aquinodermata which would be the first branch however of all of the chordates the most basal group would be the cephalochordata so that's the cephalocardia that had evolved first and then the urochord so common name for the cephalochordata is the lancelets there's an example of lancelet shown in this image the bronchiostoma key characteristics of lancelets include the blade like shape so it's very similar to the lancet so they're basically named after the the lancet the blood lancet that's used to obtain blood from from humans it's very sharp object so it's got blade like shapes and it is marine organism and it feeds by suspension as you note and as mentioned before this is one of those examples in which the adult body retains all of the characteristics of typical chordate body plan so the nortochord is retained the pharyngeal slits are not converted to any other structures the dorsal hollow nerve cord retains or remains as the dorsal hollow nerve cord and nothing is it develops into vertebrae of course because this is chordate and it has the the other basal group which is the next to evolve called the euro chordata euro chord include the tunicates these are going to be more closely related to the other chordates than the lancelets as shown in the tree so these most resemble the chordates during their larval stage so you can see the larval stage they have chordate body plan the notochord the posterior tail the dorsal hollar nerve cord the ferrents with slits so pharyngeal slits they also have an current and current siphon which allows the suspension feeding to occur so this is how it draws in the water and filters the food particles these are much more highly diver derived and they have less hox genes than other vertebrates however they do as they grow up into an adult stage they do not retain the chordate body plan as previously described so the incurrent and x-current siphon is still present just the overall body plan begins to change and you can see development of an intestine and stomach full digestive system so you can see it it becomes little bit larger and what's key here is the reason why they're called the tuna kids is because of the presence of tunic which is really tissue that surrounds the the body if we look at early chordae evolution the ancestral chordates may have resembled lancelets and this is really what the the characteristics of the lancelets and the nerve cord within the lancelet suggests that the lancelets are those the basal group so in the lancelets the hox genes that organize the brain are are expressed in the the tip of the nerve cord and these are very similar to the hox genes that organize the vertebrae brain of course the vertebrates evolved much later so here we're talking about early chordates and the evolution of chordates all the way through to vertebrates so if we look at this comparison we have vertebrae embryo and we can see these two genes bf1 and oxt up in the tip of the the the nerve column or nerve cord of vertebrae and also in the tip of the nerve cord of the lancilla embryo so upon sequencing of the tunicate genome this suggests that the genes associated with the heart and the thyroid are very common to all chordates and also the genes that are associated with the the transmission of nerve impulses these are actually unique to vertebrates which makes vertebrates little more more evolved than both the tunicate and the lancelin so these evidences suggest that the lancelets and the tudo kits considered chordates however the they are basically the evolution and the steps to the vertebrate so let's take little reminder what we just talked about we're going to be moving into the chordates in the next few segments so as mentioned we have the ancestral deuterostome the aquino dermato were discussed in chapter 33 however we're bringing it to this chapter since they are deuterostomes and then this segment really focused on the characteristics of chordates one of those characteristics is the presence of notochord at some point during their development however the cephalochordata retain the notochord the urochordata do not but both of them are considered chordates however they are not vertebrates because they have no vertebrae the next few segments we will look at the groups of vertebrates which include many other subgroups