An earthworm receives support and shape from its fluid-filled cavity. it has

an exoskeleton
an endoskeleton
a hydroskeleton
no skeleton

An earthworm receives support and shape from its fluid-filled cavity. It has Hydroskelton

Explanation:

The muscle of earthworm encloses with two layers of muscle which is circular and longitudinal. There is fluid cavity between digestive part and muscle. it is called coelom and fluid is coelomic fluid. This fluid work as hydrostatic skeleton.

It provides resistance to muscular contraction. It helps the earth worm to applying the force and change the shape. Mostly hydrostatic Skeleton organism has more or less cylindrical. This type of Skeleton found in soft bodied invertebrates.  

i'm thinking (a) to keep sand in place; ////////////

explanation: a sand fence or sandbreak, like a snow fence, is used to force windblown, drifting sand to accumulate in a desired place. sand fences are employed to control erosion, sand dune stabilization, keep sand off roadways, and to recruit new material in desert areas.

a. by using fur or clothes to trap heat energy near the body