BONES ON THE OUTSIDE?

Some animals unlike humans have their skeletons on the outside. Shells you find on the beach are examples on these animals. Such shells used to have animals living in them, and those shells protected the animals inside from external threats. An example on an animal that has a shell on the outside is the crab.

Questions:

1. What is a function of the outside skeleton for the animals that have it?
2. other than the crab, can you think of an animal that has its skeleton outside?
3. Do humans have any bones on the outside?

References:

Lievesley, T. & Herridge, D. (2003) Heinemann Explore Science Pupil book. Heinemann Oxford: Educatinal Publishers. 5-18

crab pic retrieved from:(http://media.merchantcircle.com/33263661/a008-cartoon-crab-clipart_medium.png)