700 species Theseare the most highly evolved of the molluscs, some possessing a vey efficient nervous system and eyes. The two sides of the body are symmetrical, and the well-developed head is surrounded by a circle of sucker-bearing arms. Water leaves the mantle cavity through a muscular tubular organ called a siphon, drived from the foot. The shell is often almost invisible or entirely absent. The only surviving genus with a complete external shell is Nautilus |
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