There are a variety of animals that live inside of shells. Many of the animals that reside in shells also reside in or near the water. Some animals continually upgrade their shells to larger accommodations while others remain in the same shell their entire life. Shells offer warmth and protection for the animals that use them as their home.
Turtle or tortoise shell consists of two parts. The upper half is called the carapace and the bottom half is called the plastron. The two halves are joined along the middle of the shell by a bony bridge. The turtle's shell has blood and nerve supply and they can feel pain if it is injured. There is a layer of keratin coating the shell in patches called scutes. Different species of turtles have different patterns of scutes. This is what gives a turtle shell its distinguishing features. If scutes are not properly shed or maintained they can cause a variety of shell diseases.
Crabs continually shed and regrow their shells. A crab's shell is specifically called a carapace, similar to the top half of a turtle shell. When a crab sheds its shell, it splits along the back of the shell which allows the crab to crawl out. A crab's growth cycle is different than most species with an internal skeleton. They only grow when they moult their shell. During this moulting period the crab can grow anywhere between 15 and 25 per cent. A crab is able to grow a new shell by absorbing water into its body tissue once it moults. This expands its new soft shell and in approximately three months this new shell will be completely hardened.
A snail shell is continually growing along with the snail. However, unlike a crab a snail does not shed its shell but the growth happens on the outermost part of the coil of the shell. Their shell is one solid piece in a single coil, unlike the turtle shell which is two distinct parts. A snail is different than a slug; although often confused for being similar, the distinct difference is that a slug does not have a shell.