American Scientist

To the Editors:

The excerpted section from Beautiful Geometry by Eli Maor and Eugen Jost in the March–April issue was interesting. However, the discussion of the nautilus shell warrants some clarification. The nautilus is a cephalopod, not a gastropod (such as a snail) as the authors state. Both gastropods and cephalopods are invertebrate animals, but they are in different taxonomic classes.

The shell of the nautilus is divided by partitions called septa into chambers, and only the last-formed chamber is occupied by the living cephalopod. As the cephalopod grows, it moves forward in its partitioned shell; the septa are perforated and a cord of body tissue, called the siphuncle , extends from the cephalopod back through previously occupied chambers. The siphuncle secretes gas so that the empty chambers provide buoyancy for the cephalopod, enhancing its ability to swim and float.

Wayne I. Anderson
Cedar Falls, IA