1978 年 37 巻 4 号 p. 230-239
It is reported that the Nautilus makes a diel vertical migration from a depth of about 500 m up to the near-surface of the sea. However, it is axiomatic that such a long-ranged migration within a short period of the time cannot be managed by only water-jet propulsion through the hyponome. There has been a number of early works for elucidating how the animal is possible to make this migration, but no satisfactory solution has been obtained so far. Through an anatomical observation on muscle part of Nautilus macromphalus, we could speculate that two major retractors, namely, funnel and cephalic, may be involved in downward migration. Our hypothesis is that the contraction of these two retracting muscle will create a pressure in a sea water intaken in the mantle cavity and this pressure may be transmitted (through unknown mechanism) to the siphuncle. If so, the gas contained in other chamber could be compressed and makes the specific gravity of animal increased.