AN OCTOPUS WITH ONLY SEVEN ARMS: ANATOMICAL DETAILS

A male specimen of Octopus is described which has only sevenarms: the fourth right arm is congenitally absent. Dissectionreveals that, as in normal octopuses, eight brachial nervesarise from the brain. However, right brachial nerves 2, 3 and4 are connected by unusually short lengths of the interbrachialcommissure, distal to which right brachial nerves 3 and 4 runtogether, with corresponding arteries, into the third rightarm. Hectocotylization of the latter confirms that it is thefourth right arm which is missing. Other abnormalities of thespecimen include an obviously asymmetrical funnel organ andthree pairs of conjoined suckers. An old specimen of Octopus described as having only seven armswas re-examined for comparison. This animal, too, appeared tohave no fourth right arm, but on closer inspection the extremelyshort stump of this arm was discovered. ^*Present address: National Institute for Basic Biology, Nishigonaka38, Myodaiji-cho, Okazaki 444, Japan (Received 10 August 1988; accepted 1 November 1988)