Croonian Lecture, 1973 On buoyancy and the lives of modern and fossil cephalopods

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Abstract
For about 500 million years the cephalopods have been among the most successful of marine animals. Their evolution depended on the development of buoyant chambered shells, and the lives and behaviour of modem forms are also largely dependent on the particular solutions to the problem of buoyancy adopted. There are striking similarities on the ways in which the shells of Nautilus , Spirula and Sepia are formed and used. These similarities, together with comparisons of the gross and fine structures of modem and fossil shells, give a firm base on which hypotheses about the lives of the fossil nautiloids, ammonoids and belemnoids can be made. Although many modem squid are active and swim continuously to remain in mid-water, a very large number of oceanic squid have replaced the buoyant shells of the fossil cephalopods by tissues containing large amounts of ammonium. These ‘ammoniacal’ squid, although they are not readily caught in mid-water nets, are extremely numerous for they form the principal food of the approximately 1¼ million sperm whales still living in the oceans of the world.

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