THE LOCOMOTOR SYSTEM
SKELETONS INSIDE AND OUT
Skeletal systems come in many forms. You have a skeleton inside of your body (endoskeleton) made up of bones. Insects and crustaceans have skeletal systems on the outside or their bodies (exoskeletons) that are made of hard plates. 
Organisms like starfish do not have bones or plates. They have skeletons made up of fluids inside of tubes within their bodies. The fluid skeletal systems are called hydrostatic. All animals that live outside of the water need some kind of skeletal system to support or protect them.
WHAT DOES THIS SYSTEM DO?
We already hinted at the purpose of a skeletal system. Protection and support are the two big reasons that organisms have skeletal systems. In your body, the skeleton works very closely with the muscular system to help you move. Without the bones of your skeleton, you would be a blob of water-filled tissues. The bones create a framework to which your muscles and organs can connect. Your skeleton also plays a role in protection, especially in your head. The bones of your skull protect your all-important brain. Your ribs protect most of your internal organs from impact as well. Other animals with exoskeletons receive obvious protection from their skeleton. Crabs and insects have hard shells made of chitin to protect their entire bodies. 
INTERACTING WITH OTHER SYSTEMS
Your skeletal system does not work alone. We already mentioned the interaction with your muscular system. Muscles connect to your skeleton and they contract and move the skeleton along. Your skeletal system is made up of cartilage and calcified bone that work together. They help the process of movement happen in a smoother manner. The calcified bones of your skeleton also work with the circulatory system. Marrow inside of your bones helps produce the cells inside of you blood. Both red blood cells and white blood cells are created in your bones.
MUSCULAR SYSTEM - MEAT ON THE BONES
Many advanced animals have muscular systems. You know you do. Did you know that your muscular system is made up of three different types of muscular tissue? You have smooth, cardiac, and voluntary muscle tissue in your body. Smooth muscle is muscle you rarely control such as the muscle in your intestinal tract. Cardiac muscle is very specific tissue found in your heart. Voluntary muscle is the muscle that helps you move. All of those tissues add up to a muscular system that is found through your body. There is more to the muscular system than the muscles that help you move.
WHAT DOES THIS SYSTEM DO?
The big purpose of the muscles found in your body is movement. We could be talking about the movement of your legs while you walk. We could be talking about the beating of your heart. We could also be talking about the contraction of a very small blood vessel in your brain.
You have no control over most of the muscular system. You do control the voluntary muscle in your arms, legs, neck, and torso. You have little or no control over the heart or smooth muscle. Those other muscles are under the control of the autonomic nervous system (ANS).
INTERACTING WITH OTHER SYSTEMS
We just teased the fact that your muscular system is closely connected to the nervous system. That makes sense since you usually have to think before you can move. Even though thinking is not always involved, the neurons of the nervous system are connected to most of the cells in your muscular system. You have smooth muscles that line your digestive system and help move food through your intestines. Smooth muscle also surrounds your circulatory system and lymph system. Those muscle tissues are spread throughout your body and are even involved in controlling the temperature of your body.
MUSCLES HELP YOU MOVE
The main parts of your voluntary muscular system include the muscles, and tendons. Tendons connect your muscles to your bone at insertion points.
When the muscle shortens, the bones are pulled closer together. Muscles called flexors force your joints to bend. Muscles called extensors cause your limbs to straighten. A bicep is a flexor and the triceps are extensors. You may have also heard of ligaments. They are batches of connective tissue that bind bones to each other. Muscles, tendons, and ligaments can been found working together in almost all of your joints.
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