Some one with a more scientific bent can explain this in more detail. I'm an elemetary teacher, so my knowledge is more aimed at simple explanations! lol!
An insect has 6 legs, 3 body sections and an external "skeleton". If it doesn't have one of these things it isn't an insect. The skeleton isn't really bones. It's that hard "shell" around the outside of the insect. There are no bones or a hard shell inside the insect's body. If the hard shell is punctured or cut, it doesn't heal up.
A snake is a reptile. A reptile is cold blooded, has a back bone and tough, scaley skin. Reptiles have bones inside their bodies like we do. The outside of a snake is skin, that if punctured or cut, can heal. A turtle is also a reptile. It has bones in it's body. Its backbone is modified to form a shell. (I know, it may not stay classified as a reptile, but I'm trying to keep it simple here! lol!)
A worm is neither a reptile or insect. I don't off hand know where it belongs in the taxonimy, not important. It doesn't have bones. Worms have a hydrostatic skeleton, which essentially is a bag of water, that helps it keep its shape. It moves my contracting its muscles. Their outside is skin. Like our skin, it is elastic and resists cutting. If it is cut, it will heal.
lol! I think that is about a semester's worth of science lessons!
-----
0.1.0 ornate box turtle
1.0.0 eastern box turtle
1.0.0 Yellow belly slider
0.1.0 Red belly cooter
0.1.0 Australian shepard
1.11.0 chickens
1.0.0 Dutch(rabbit)
3.2.0 children (do I still count the married ones?)
1.0.0 husband