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Lithoderms (Phylum Lithoderma, from the Greek: λιθο-, "stone", and δέρμα, "skin")

Lithoderms are characterised by the possession of a hard mesodermal-skeleton made up of a combination a chitin like shell covered with overlapping carbonate and sillicate plates, as well as a segmented body with appendages on each segment. They have a dorsal heart and a ventral nervous system. The coelom is largely filled in with tissue and reduced to a system of narrow canals. Lithoderms have a complete digestive system with both a mouth and anus.

The skeletal structure of Lithoderms is a complex rigid mesodermal skeleton consisting of a thin skin covering a layers of tiny overlapping plates of sillica and carbonate in a chitin like polysaccharide matrix, which forms a rigid support contained within tissues of the organism and strengthening them against attack by predators. The calcite is rich in magnesium oxide, confering a higher skeletal density and a stronger, more resistant skeleton. The skeleton takes the form of jointed plates and rings on the appendages that segmenting them by joints.

Lithoderms generally have 2-6 tagmata. All forms have a "head" composed of a presegmental acron that usually bears the eyes (compound and/or simple), followed by several fused segments, all closely fused, which usually support several specialized appendages for feeding, sensory reception, and defense. This is followed by several segmented or fused tagmata with from 2 to 12 appendages per segment used primarily for locomotion, and occassionally feeding, sensory reception, and defense. In some forms there is an appendageless segmented "tail".

Aquatic Lithoderms use gills to exchange gases. These gills have an extensive surface area in contact with the surrounding water. Terrestrial Lithoderms use book lungs, or gills modified for breathing air The gill chambers in terrestrial crabs sometimes have two different structures: one that is gilled and used for breathing underwater, and another specially adapted to take up oxygen from the air (a pseudolung).

Lithoderms have a closed circulatory system. Blood containing hemoglobin is propelled through the capillaries of the gills by two gill hearts (also known as branchial hearts). A single systemic heart then pumps the oxygenated blood through the rest of the body.

The phylum is divided into three orders: icosapoda, dodecapoda, and lithooctapoda.