Six living specimens of Nautilus macromphalus, which was trapped on June 20, 1976, have been kept alive for exhibition at Yomiuri Land Marine Aquarium since July 22. The results of observations on maneuvering, feeding, copulating and spawning behaviors as well as on egg case and sexual dimorphism in external morphology are summarized as follows : (1) N. macromphalus moves and reacts to external stimuli very gently and slowly. The animal usually fastens itself to the substratum in daytime. The funnel is a sole organ for foreward and backward travelling as well as steering. A peculiar pendulum-like movement during these motions is observed.(2) One or two pairs of the ventralmost tentacles are hanging below towards the bottom, while two pairs of short tentacles in front of and behind the eyes are always kept in horizontal posture.(3) N. macromphalus never reacts to living prey but easily accesses to food given with a forceps or scavenges food lying on the bottom.(4) In copulating, tentacles of both sexes tangle with each other, and a coupling pair does not oriented in a linear position but forms an open L-shaped.(5) The "spadix" is a bulbous "lily-tuber-like" organ with which the male contacts to the female at buccal area. After copulation, the female is found carrying a string-like organ (probably sperm-bulb).(6) The female has pronounced tentacles on the inner brachial crown. These tentacles are carried by bifurcated lobe forming a hand-shaped organ. This organ is less pronounced in the male.(7) In laying egg capsule, female manipulates the labial tentacles. The egg capsule is ovoidal or thumb-shaped laid horizontally or obliquely vertically adhered on the substratum. The outer skin of the capsule is as tough as something cartilaginous. Among 15 eggs, only four seemed to be normal but none developed in the aquarium.
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