Class Amphineura
Class Amphineura comprises two subclasses of primitive forms of mollusk: aplacophora and polyplacophora (chitons). The first are small wormlike animals that live in moderately deep, ocean water. These comprise about twenty genera. The latter are the more common chitons, with eight serially arranged dorsal plates.
Chitons, or "Armadilos of the ocean", are restricted to marine life. Chitons have a bilaterally symmetrical, elongated and flattened body, with a broad foot anterior mouth and posterior anus. The strong muscular foot can clamp onto rocks in shallow waters allowing the radula to scrape the rocks for food. Between the mantle and foot there are a series of gills that extract oxygen from the water. The head lacks eyes or tentacles but the mouth has a well-developed radula. A ladderlike nervous system is formed of four longitudinal nerve cords connected to an anterior nerve ring.
The shell consists of eight flat, overlapping valves or plates held together by a muscular tissue called the girdle. Between the plates are joints, which permit the chiton to roll up in to a ball like a pill bug or an armadillo.