Nautilus macromphalus Sowerby when freshly caught was close to neutral buoyancy having a weight in sea water of about 0–2% of its weight in air. The animals without their shells varied considerably in density but the volume of the shell was an approximately constant fraction of the total volume of the whole animal and whole animals were brought approximately to the same density by havingmore or less liquid inside the chambers of the shell. About 80 % of the gas space in the shell was used to support the weight of the shell itself in sea water.In an adult animal the centre of buoyancy was found to be about 6 mm above the centre of gravity, which made the animal very stable in its natural swimming position, a couple of about 350 g. cm being required to turn it through 90°. The pearly partsof the chamber walls were impermeable to sea water but the chalky and horny siphuncular tubes joining the septal necks were very porous. The most newly formed tenor so chambers were the only ones to contain liquids in appreciable volume and theydid this in diminishing amounts from the newest to the oldest. The watery liquids found within the chambers were always hypotonic to sea water and sometimes markedly so; they contained principally sodium and chloride ions. One animal was in the process of forming a new chamber, this incomplete chamber was completely full of liquidwith an osmolarity close to that of sea water but differing in composition from seawater.

Squids (teuthoids) fall into two distinct groups according to their density in sea water. Squids of one group are considerably denser than sea water and must swim to stop sinking; squids in the other group are nearly neutrally buoyant. Analyses show that in almost all the neutrally buoyant squids large amounts of ammonium are present. This ammonium is not uniformly distributed throughout the body but is mostly confined to special tissues where its concentration can approach half molar. The locations of such tissues differ according to the species and developmental stage of the squid. It is clear that the ammonium-rich solution are almost isosmotic with sea water but of lower density and they are present in sufficient volume to provide the main buoyancy mechanism of these squids. A variety of evidence is given which suggests that squids in no less than 12 of the 26 families achieve near-neutral buoyancy in this way and that 14 families contain squids appreciably denser than sea water [at least one family contains both types of squid]. Some of the ammonium-rich squids are extremely abundant in the oceans.

The Plymouth Laboratory Cuttlefish are amongst the most remarkable of all animals, and we can well understand why Paul Bert (1867) was inspired to undertake a general study of Sepia officinalis L. This he vowed not to leave until the physiology of Sepia was as well understood as that of the frog. Bert did little work on the cuttlebone; he analysed the gas contained in it and expressed the opinion that this gas would vary according to circumstances, in the same way as the gas in the swim bladder of fish, then the subject of the beautiful experiments of Armand Moreau (1876). The lack of interest of physiologists in the cuttlebone, one of the most common objects on our sea shore, is perhaps excusable, for the cuttlebone is dead. In this and the following three papers we show that, although dead, the cuttlebone is not unchanging and that the cuttlefish can use it as a variable buoyancy tank. This extraordinary animal can change its density quickly and does so in response to changes in light intensity. Finally we show that liquid is probably moved in and out of the cuttlebone by an osmotic mechanism, and not by changes in the internal gas pressure which is the method used in the swimbladder.The cuttlebone has a special interest in that it is closely related not only to the shells of the living Nautilus and Spirula, but also to the shells of the fossil Nautiloidea, Ammonoidea and Belemnoidea (see Naef, 1923;Morton, 1958). These animals dominated the Palaeozoic and Mesozoic seas and their evolution from a crawling to a free-swimming life was probably determined by the use of the shell as a buoyancy device. We hope in the light of new knowledge of the cuttlebone to see more clearly how these important animals lived. The chambers of the cuttlebone contain liquid as well as gas, and the cuttlefish changes its density and posture by varying the amounts of liquid which the chambers of the bone contain. If the fossil cephalopods could also have done this their behaviour must have been very different from that postulated on the usual assumption that their chambered shells were completely filled with gas.

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