Usually the entire body of Crabs is covered with a hard, calcified armour.
The Hermit Crab however lacks this protection on the hindmost part of its body, the abdomen, which is soft and vulnerable.
It protects its abdomen, which contains such important organs as the liver and the gonads, by inserting it into a gastropod shell
For this purpose the abdomen is bent, so that it fits in the curvature of the snail-house.
The outer bend of the abdomen bears some little appendages which enable the crab to hold on to the inside of the shell. The reduced 4th and 5th pairs of legs serve the same purpose
When walking the animal drags its house along. In water the weight of the shell is diminished by the upward pressure and so the Hermit Crab, in spite of its burden, can zealously run about
In case of danger the Hermit Crab withdraws into the shell as deep as possible. In the shell there is no room for two big pincers. Therefore the Hermit Crab has but one. When hiding in the shell it uses this pincer to guard the entrance.
When the crab grows and does not fit in its shell any longer, it looks for a bigger one. The original occupant, if still present, is picked out. Then, quickly and nervously, the crab moves over into its new home.
Often a colony of little polyps grows upon the surface of a Hermits's shell and forms a rough, brown layer on it. A bald patch indicates the spot that touches the substratum when the shell is being dragged along.
The polyps profit by the crumbs of the Hermits's meals and in turn the hermit profits by the protection given by the nettle-cells of the polyps.
Sometimes the Hermit Crab bears Sponges or Sea-anemones on its shell. Besides giving protection these guests give some useful camouflage. When moving into a new shell the crab can remove these from the old one and transplant them.
Pious men used to withdraw into the desert in order to service God in strict solitude. They used to live in a cave or in a little house all by themselves. They were called "hermits" ("eremos" is Greek for "desert"). Hence this name for the funny crab that lives all alone in its self-chosen hermitage.