 Animals like the butterfly, the crab and the scorpion belong to the phylum Arthropoda. This is a vast phylum over 80% of known animal species constitute this phylum Arthropoda. So let's first understand what the name Arthropoda means. So Arthropoda means joint and poda means feet or appendages. So Arthropoda have jointed appendages. What do we mean by that? So first of all let's understand what an appendage is. An appendage is an attachment to the body. In case of the arthropods it can be the legs, the antennae etc. And what do I mean by jointed? They have joints in them. So this for example is the leg of an insect. So insects are also types of arthropods right? So if you see this leg is not one continuous piece. It is made up of all these separate smaller pieces. Each of them is joined to the previous one and the next one. So the joints are responsible for keeping the leg together. So in this way all the other appendages like the antennae are also jointed. So arthropods have appendages with joints in them. Another characteristic of arthropods which is applicable to all of them is the exoskeleton. So in order to understand what the exoskeleton is let's bring back our scorpion. So if you look at the scorpion it looks quite strong as if its body is covered with an armour. In fact it is kind of. So the outer covering is called the exoskeleton. Skeleton is something that is responsible for the rigidity and the firmness of an animal body. So for us we have a skeleton made up of bones which is inside. So that's an endoskeleton whereas for the arthropods the skeleton is outside the body protecting it. Hence it's called exoskeleton. Exo means outside. Now what is the exoskeleton made of? The exoskeleton is made up of chitin. So they have a chitinous exoskeleton. So you might remember that chitin is a polysaccharide that is found in the cell walls of fungi. Is it the same thing? Yes it is. It's the exact same chemical that is also found in the exoskeleton in arthropods. Since the arthropods are so varied so are their respiratory organs. So let's take a look at some of them. First we'll look at aquatic arthropods. The arthropods that live in water. So you know that aquatic animals have gills as their respiratory organs. You know fish breathe with the help of gills right? Similarly aquatic arthropods many of them have gills. For example if you look at this organism it's kind of like a prawn it's actually a krill. So these right here these are the gills. There are some other arthropods which also have gills but their gills have a specific shape. So you see right here these are gills but it also looks like a stack of books. Hence they are called book gills. This was about the aquatic arthropods. What about the arthropods which live on land? Those arthropods are called terrestrial arthropods. Now you know that many land animals including us breathe with the help of lungs. Well so do many terrestrial arthropods. So this right here is a spider and this is its respiratory organ. These are actually lungs but again like this one also looks like a stack of books. Hence they are called book lungs. And the last type of respiratory organ is actually quite common among arthropods especially insects. So these constitute of a network of tubes. You see all these tubes over here. All these tubes are called trachea. Singular trachea and each of these tubes carry only air from the outside. So you see these tiny openings on the sides of the body. These are spiracles. So the arthropods take in air through the spiracles and also give out air through the spiracles. The difference of the tracheal system from the book lungs or gills or book gills are if you remember in gills and lungs it is the blood which takes up the oxygen. So the oxygen dissolves in the blood and then blood transports the oxygen throughout the body. Whereas in the tracheal system there is no blood involved in respiration. The oxygen comes into the tubes and goes directly to all parts of the bodies without the help of the blood. So isn't this an amazing variety just in terms of the respiratory system? Stay tuned in the next video we will look at more characteristics of the arthropods.