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Lithostrata (stoney scales)

Lithostrats are a phylum of elongted flattened, benthic jawless aquatic vertebrates marked in their arrangement of four plates of silicate scales which form two large dorsal shields and two large ventral shields, and the series of scales arranged in various patterns on the sides of their bodies. The scales have three layers composed of dentine, aspidine and a honeycombed cancella middle layer. They have a cartilaginous skeleton. and a boney, jawless skull. They have a rudimentary brain to integrate signals from sensory organs such as eyespots.

There is an opening on the dorsal surface between the dorsal shields, connected to the pharynx and gill chamber, which serves as both the olfaction and the intake of the respiratory water. There are up to 45 gill openings. The mouth opening is situated on the ventral side of the flattened head.

They have a circulatory system with a heart that also functions as a kidney, and complete digestive tracts, but food is mostly transported through it by using the cilia that cover its inside surface. They move via undulating locomotion, aided by the gill plates, which are drawn out to form wing-like extentions. Depending on species and age, individuals range from almost microscopic to over 20 m long.

Reproduction is by sexual spawning.