 What if I were to tell you that you looked something like this at some point in your life? Yeah, you heard me right. All of us looked something like this when we were growing as embryos in our mother's bodies. This structure or something that resembles this structure is common for all animals classified under the phylum Chordata, that is all Chordates. Animals are classified as Chordates if they have four specific characters. They are the notochord, the dorsal hollow nerve cord, pharyngeal gill slits and a post anal tail. A notochord is a rod-like structure that usually involves in supporting the animal's body. The nerve cord is made up of nervous tissue and is located above the notochord, hence the name dorsal nerve cord because dorsal means above or on top. Gills slits are openings on the sides of the animals. The post anal tail is found towards the end of the animal's body and is usually involved in movement. Take a look at yourself. Do you have a post anal tail or do you have gills? How are we Chordates then? An important thing to remember here is that Chordates need not have these structures throughout their life cycle. If an animal has all four characters at some point in its life, it is enough to classify it as a Chordate. Now Chordata is a huge phylum. It is classified into three main sub phyla, Eurochordata or Tunicates, Cephalochordata or Cephalochordates and Vertibrata. In tunicates such as salp, which are marine organisms, the notochord is present only in the larval stage, that too in the tail. This fits because the term Euro actually means tail. So in tunicates the notochord is present only in the larval tail. Cephalochordates such as Lancelots live in shallow waters. In cephalochordates the notochord is present in adults as well, that means it is present throughout the organism's life. The notochord often extends from the head to the tail in many species. In vertebrates the notochord is present in the embryonic stage but is later replaced with the vertebral column or the backbone. If you put your hand to the middle of your back the heart structure that you feel there that's your backbone because all humans are vertebrates. Now vertebrata again is a huge sub phylum. It is divided into two main divisions, agnata which are jawless organisms and gnatastomata which are jawed organisms. So by jaw I mean the muscular jaw that we have. Agnata don't have that while gnatastomata have jaws. There is one class classified under agnata which is cyclostomata. Cyclostomata such as hag fish and lampreys are marine organisms. They have a circular sucking mouth because they don't have jaws. They live as parasites on other fishes but their body is not covered with scales and nor do they have any fins. Gnatastomata is classified into two main categories. Pisces which are animals that pair fins and tetrapods which are animals that have limbs. Now Pisces are exclusively aquatic organisms. Tetrapods may be aquatic or they may be terrestrials which means they can live on land. Let's take a closer look at Pisces and tetrapods. There are two classes of Pisces, chondrichthys and osteichthys. Chondrichthys such as sharks and stingrays have a skeleton that is made up of cartilage. Now cartilage is a type of soft tissue. In chondrichthys the notochord is present in the adult stage as well. Because they're aquatic organisms they have gill slits through which they breathe. Their body is covered with scales and in many organisms these scales are sometimes modified into teeth and these organisms are often predatorous. So they are cold-bredded organisms which means that they lack the ability to regulate their internal temperature according to the external temperature. Osteichthys on the other hand like seahorse and clownfish have a skeleton that is made up of bone. While chondrichthys have a skeleton that is made up of cartilage, osteichthys have a skeleton that is made up of bone. They're also aquatic animals so they breathe through gill slits. Their body is covered with scales and is more streamlined to help in swimming. They have something called air blooders. So because of the presence of air blooders these animals are buoyant so they need not keep swimming constantly to avoid sinking. Air blooders are noticeably absent in chondrichthys. So animals like sharks need to keep swimming constantly to avoid sinking. Tetrapods are mainly terrestrial organisms but there are of course a few exceptions. There are four main classes of tetrapods amphibia, reptilia, avis and mammalia. Amphibia includes all amphibians like salamanders and frogs. They can live in water and land. They have a soft moist skin that is not covered with scales. They can breathe through their skin but they also have lungs through which they can breathe. They are also cold blooded animals. Reptilia includes all reptiles like crocodiles and snakes. So these animals are animals that move in a creeping or crawling manner. This fits because the word reptilia is derived from the Latin word repper which means to creep or crawl. Their skin is dry and is covered with scales and often animals like snakes shed their skin as they grow. Now reptilia are also cold blooded organisms but this is where the feature of cold bloodedness ends because avis and mammals are not cold blooded. Then what are they? Avis are actually birds. Birds like peacocks have a feathery body. Their body is covered with soft feathers. They have wings that help them fly. So their bones are hollow which means that they have space in between them and the space is filled with air cavities and these air cavities also help in flight. Now like I just said avis and mammals are not cold blooded. They are called warm blooded animals. This is because they can regulate their internal body temperature according to the external temperature. Mammalia includes all mammals like elephants and us humans. A basic characteristic of mammals is that the body is covered with hair and female mammals have mammary glands and females often give birth to young ones. So they are called viviparis. All animals that lay eggs are called oviparis but animals that give birth are called viviparis. So because of the presence of mammary glands females produce milk which is used to nourish young ones. Mammals are also warm blooded like birds and they have adapted to a variety of locomotive methods like walking in elephants and swimming in whales. Majority of mammals do give birth to young ones but there are of course exceptions. One example of an exception is the platypus which instead of giving birth actually lays eggs. Pretty cool isn't it? So now we can safely say that all vertebrates are chordates but are all chordates vertebrates? Why or why not?