Skeletal System

 The skeletal system is important. It allows movement, protection, support, mineral storage, and blood cell formation.

The earthworm's skeletal system is different. It has no bones. It has a hydrostatic skeleton, which means instead of bones, it has a fluid-filled cavity surrounded by muscles. We'll go into detail about muscles later. 

-This means the worm has no ribs, no arms, or no legs. But that's obvious. This also means the worm has no calcium nor requires its intake. This differs from all of the other organisms.

-A disadvantage to not having muscles is that it has no strength. You don't see a worm doing push-ups. But, that means its weak, and can't really defend itself. 

-The worm also has this thing called a coelom. If this coelom gets injured, the worm dies. One mistake, and boom, you're dead.

-An advantage, however, to not having a skeleton is that movement is easier. Shocking, right? Actually, a worm is more flexible, and can move with more ease. This is helpful in many ways. It can get out of trouble in a matter of minutes, which comes in handy when you don't have muscles to fight.

-Another advantage is that the healing process regenerates quickly. The worm doesn't have to wait too long for some of its body to come back. This is helpful because, it can get back to normal quicker.

The crayfish's skeletal system is next. 

crayfish's exoskeleton

-The crayfish has a antennae, which is used for senses. This is an advantage because it can sense danger early, and can run away.

-Like the worm, the crayfish is segmented, but in a different way. Its tail is segmented. This allows flexibility, which is helpful when it comes to swimming.

-The crayfish has an exoskeleton, which is made out of chittin, protein, and lipids. This exoskeleton has its goods and its bads. One good thing is that it can protect the creature. On other creatures, an injury will come in first contact with our skin, but on an exoskeleton, the injury hits the bones first.

-The crayfish has a cheliped. This is its claws.These work as hands, as well as scissors, which is an advantage because it can be used as defense.

-Since the crayfish has bones, it has calcium. This calcium can be used to make muscles contract, have blood clot, and allow nerves to send and recieve messages, kind of like ours.

-More about exoskeletons: The exoskeleton limits the size of the organism, which is a bad thing. If you need to be big, to scare other organisms, and not be eaten, this is a disadvantage.

-Also, this exsoskeleton weighs. The more heavy it is, the more weight the organism has to carry.

-But, on the bright side, the exoskeleton provides homeostasis for the organism. It keeps it warm or cold, and that's definitley an advantage.

Next is the grass frog. 

-The grass frog has an endoskeleton. This means the skeleton is underneath the skin.

-The frog has a hollow, flat skull bone. This makes it easier to hide, and provides a better eyesight.

-The frog's skeleton differs from the human's because it doesn't have a neck. That means it cannot turn to look behind it.

-The frog has no fibula. Instead, it has a tibiofibula. This is an advantage because they can jump farther. If it can jump farther, it can run away, and avoid danger.
-A disadvantage to being a frog is that it can only jump. It cannot run in use of danger, making its 'getaway' slower than normal.

-The frog also has a square pelivs. Ours is circular. This allows more movement because it slides up, and down the spine, making jumping easier. 

Next is the white rat. 

-The rat has an endoskeleton. This is helpful in many ways. First, the mandible or maxialla are used in many ways. They are the bones that form the jaw. This allows chewing, which allows defense, eating, and comes in handy when you need to cut things. This is also used to chew threw things that are really hard.

-The rat has a caudal vertabrae, which make up the tail. This tail is used for balance, and homeostasis. A disadvantage about having a tail is that it can get caught in places, and it can be easier to get caught by prey. Also, its another bone that can be broken. Don't want that happening.

-Along with a tail, the rat uses its low pelvis for balance. This is an advantage because it needs to walk, and run.

-The rat has five vertabrae:

• cervical- makes up the neck of the rat. 
• thoratic-  holds the cervical and lumbar vertabrae together.
• lumbar- hold the thoratic and sacral vertabrae together.
• caudal- this is the part where the rest of the skeleton is connected to the tail.
• sacral- holds lumbar, and caudal vertabrae together.

Final is the human skeleton. 

-The human skeleton contains calcium. This calcium is present so that our bones stay big and strong. It also makes sure that muscles contract, blood cells clot, and nerves send messages.

-We have a bone called a patella. The patella allows movement. If it weren't there, our leg would be a straight, long, stick, and we wouldn't be able to move very comfortably. It is also an advantage because it allows us to run, jump, and skip. This comes in handy in many ways. Like performing a daily task, or a complicated one. This is a disadvantage because it can break, like any other bone. The worm has no patella, which means it has one less bone to break, or none at all.

-We also have phlanges. The phlanges in our hands let us grab things, which was only recently evolved. The phlanges in our feet are used for balance. Without them, we would not be able to walk. This an advantage because we need our fingers for everything: eating, climbing, etc. Without them, it would be harder to maintain things, and capture prey.

-Compared to the frog, the human hand is a lot less thin, and the frog has less bones. The frog also can't grab things, which is why you can see humans are more recently developed than frogs.