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By Monks N., Palmer P.

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In contrast, squids, lacking a chambered shell, usually weigh more than the water they displace, so tend to sink, and must swim continuously to stop sinking. 58 1 AMMONITES As described earlier, nautiluses build their chambered shells throughout their lives, adding new chambers as they grow. It is the siphuncle that does the work of emptying the chambers. The siphuncle has a semi-permeable membrane, which means that it only allows small molecules, like gases and water, to cross it. Larger molecules, like salts, sugars and proteins, cannot.

The shell can be thought of as being under a constant 'crushing attack' from the surrounding 54 &) AMMONITES water. The thinness of the shell and the lack of structural support between each septum, means the shell has little extra strength to resist the additional forces from a predator's bite. The shell could be thicker, but only by losing some of the empty space inside, and so with a loss of buoyancy. In short, the shell can be thick and strong but heavy, or thin and weak but buoyant, but not both.

By alternating between these two actions, nautiluses generate what is called a ventilation cycle, continually bathing the gills in fresh water from which oxygen can be extracted. Just as importantly, the jet produced can be used for swimming, The retractor muscles are attached to the shell at certain points where they leave distinctive 'scars'. These scars can be seen in some fossil nautiluses and ammonites, indicating that they may have been able to produce a similar ventilation cycle. As the nautilus grows, it moves itself forward, sealing off part of the old living chamber with a new septum and then filling it with gas.

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