The invertebrate mites are arthropods with a chitinous exoskeleton and jointed limbs. Arthropods are invertebrate animals with a chitinous exoskeleton, segmented bodies, and jointed legs. Classified within the arthropods, isopods have a chitinous exoskeleton and jointed limbs. It has a chitinous exoskeleton of hard plates held together with flexible membranes. The resulting material increases the rigidity of an insect's chitinous exoskeleton. Molluscs have a structure called a radula which bears a ribbon of chitinous teeth. In arthropods, the hypodermis is an epidermal layer of cells that secretes the chitinous cuticle. The walls of this have chitinous ridges, teeth and calcareous ossicles. The radula is a chitinous ribbon-like structure containing rows of microscopic teeth. In certain insects and molluscs, the gizzard features chitinous plates or teeth. The rostellum is armed with two rows of chitinous spiny hooks. The enzyme chitinase produced in the stomach helps digest the chitinous cuticle of arthropod prey. The pupae have chitinous spurs near the base of the forewings. The body has a chitinous cuticle that is moulted as the animal grows. The body is segmented and covered in a chitinous cuticle.