Muscular System:
They do not have any muscles but instead have a jelly-like material between the epidermis and endoderm called the mesenchyme.
Skeletal System:
Their entire bodies are made out of a network of protein fibres called Spongin and spicules. They help keep the shape of the sponge.
Spicules are made out of Calcium Carbonate (limstone) or silica which is glass.
Example of a Spicule:
A Study In Biology 