Nautilus

Nautilus. The term nautilus, meaning simply "the sailor," was applied by the ancient Greeks to the genus of eight-armed cuttlefishes or octopods which is now known as the paper nautilus, and whose scientific name is Argonauta (see Cephalopoda).

This animal is not uncommon in the Mediterranean, and from its habit of floating at the surface attracted the attention of the fishermen and sailors of the Aegean Sea from the earliest times. The popular belief that the expanded arms are raised above the water to act as sails and that the other arms are used as oars was not based on any actual observation of the living animal, and it is now known that although the animal floats at the surface it does not sail, the expanded arms being applied to the exterior surface of the shell, which is secreted by them. The eggs are carried in the shell, and as this structure is entirely absent in the males, there is good reason to conclude that the habit of carrying the eggs and using one pair of arms for that purpose gave rise to the modification of those arms and the secretion of the shell by them. Huxley once expressed the truth of the matter with characteristic felicity in the remark that if the shell of the Argonaut is to be compared to anything of human invention or construction at all, it should be compared, not to a ship or boat, but to a perambulator.

The shell of Argonauta (see fig. i) is spirally coiled and symmetrical, and thus bears a remarkable resemblance to the shell of the pearly nautilus and the extinct ammonites, especially r. a.

FIG. I. - The Argonaut in life. (After Lacaze-Duthiers.) Tr, Float; Br.a, ventral or posterior arms; Br.p, dorsal or anterior arms; V, the expanded portion of them, once called the sails; B, the beak; C, the shell; En, the funnel.

as it is like that of the pearly nautilus coiled towards the dorsal or anterior surface of the animal. It is ornamented by ridges and furrows which pass in transverse curves from the inner to the outer margin of the coils. The outer margin or keel is somewhat flattened and the whole shell is compressed from side to side. It differs entirely from the shell of the pearly nautilus in the absence of internal septa and siphuncle and in the absence of any attachment between it and the body. It is in fact entirely different in origin and relations to the body from the typical molluscan shell secreted by the mantle in other Cephalopods and other types of Mollusca. It is a structure sui generis, unique in the whole phylum of Mollusca.

The only description of the living animal by a competent observer which we have is that of Lacaze Duthiers, made on a single specimen on the Mediterranean coast of France, and published in 1892, and even this is in some respects incomplete. The specimen after capture was carried in a bucket, and became separated from its shell. When placed with the shell in a large aquarium tank the animal resumed possession of the shell and assumed the attitude shown in fig. i. The shell floated at the surface, doubtless in consequence of the inclusion of some air in the cavity of the shell. It is not known with certainty that the animal is able in its natural state to descend below the surface; the specimen here considered never did so of its own accord, and when pushed down always rose again.

The siphon or funnel is unusually large and prominent, and is the chief or only organ of locomotion, the water which is expelled from it driving the animal backwards. The arms are usually turned backwards and carried inside the shell, to the inner surface of which the suckers adhere, but one or two arms are from time to time extended in front. This does not apply to the dorsal arms which are applied to the outside of the shell, and the expanded membrane of these arms covers the greater part of its surface. The dorsal arms are turned backwards, and each is twisted so that the oral surfaces face each other and the suckers are in contact with the shell. The membrane or velum is thin, and is really a great expansion of a dorsal membrane similar to that which is found along the median dorsal line of the two posterior arms. The suckers of the originally posterior series of each dorsal arm lie along the external border of the shell, and the arm with its two rows of suckers extends round the whole border of the membrane, the arm being curved into a complete loop, so that its extremity reaches almost to the origin of the membrane near the base of the arm, the extremity being continued on to the internal surface of the membrane. The external row of suckers, originally the posterior row, are united by membrane which is continuous with the velum. The smaller suckers on the more distal part of the arm, which extends along the edge of the shell-aperture, are quite sessile. In the figure of Lacaze-Duthiers (fig. I) the suckers appear to be turned away from the shell, but this is erroneous. A figure showing the natural position is given in the Monograph of the Cephalopoda in the series of Monographs issued by the Zoological Station of Naples.

The animal described by Lacaze-Duthiers lived a fortnight in captivity, during which time it devoured with avidity small fishes which were presented to it, seizing them, not by throwing out all the ventral arms, but by means of the suckers near the mouth.

Judging from these observations, Argonauta is a pelagic animal which lives and feeds near the surface of the ocean. Several species of Argonauta are known, distributed in the tropical parts of all the great oceans. The male is much smaller than the female, not exceeding an inch or so in length. It secretes no shell and its dorsal arms are not modified. The third arm on the left side, however, is modified in another way in connexion with reproduction.

Argonauta is one of the Cephalopods in which the process known as hectocotylization of one arm is developed to its extreme degree, the arm affected becoming ultimately detached and left by the male in the mantle cavity of the female where it retains for some time its life and power of movement. The hectocotylus or copulatory arm in the Argonaut is developed at first in a closed cyst (fig. 2), which FIG. 2\. - a, Male of Argonauta argo, with the hectocotylized arm still contained in its enveloping cyst, four times enlarged (after H. Muller). b, Hectocotylus of Tremoctopus violaceus (after Kolliker).

afterwards bursts, allowing the arm to uncoil; the remains of the cyst form a sac on the back of the arm which serves to contain the spermatophores.

The animal known as the Pearly Nautilus was unknown to the ancient Greeks, since its habitat is the seas of the far East, but in the middle ages, when its shell became known in Europe, it was called, from its superficial similarity to that of the original nautilus, by the same name. It was Linnaeus who, in order to distinguish the two animals, took the name "nautilus" from the animal to which it originally belonged and bestowed it upon the very different East Indian Mollusc, giving to the original nautilus the new name Argonauta. Zoological nomenclature dates from Linnaeus, and thus the nautilus is now the name of the only living genus of Tetrabranchiate Cephalopods. A detailed description of this animal is given in the article Cephalopoda (q.v.); it is only necessary to add here a brief account of its mode of life and habits.

Four species are known from the Indian and Pacific oceans; they are gregarious and nocturnal animals living at some depth and apparently always on the bottom. The natural attitude of the animal as represented by Dr Willey is with the oral surface downwards, the tentacles spread out, and the shell vertical. The chambers of the shell have no communication with one another nor with the siphuncle, they are air-tight cavities and filled, not with water, but with a nitrogenous gas. This necessarily very much reduces the specific gravity of the animal, but it is still heavier than the water and does not seem capable of rising to the surface any more than an octopus. Nautilus is rather abundant at some localities in the East Indian Archipelago, for example at Amboyna in the Moluccas. In1901-1902Dr Arthur Willey of Cambridge University spent some time in that region for the purpose of investigating the reproduction and development of the animal. He stationed himself at New Britain, known to the Germans as Neu Pommern, an island of the Bismarck Archipelago off the coast of Papua. The natives of this island use the nautilus for food, capturing them by means of a large fish-trap similar in construction to the cylindrical lobster-traps used by British fishermen. Fish is used for bait. Dr Willey found the males much more numerous than the females; of fifteen specimens captured on one occasion only two were females. He kept specimens alive both in vessels on shore and in large baskets moored at the bottom of the sea. He found that when they were placed in a vessel of sea water numbers of a small parasitic crustacea issued from the mantle cavity. Some of the females laid eggs in captivity, but these were found not to be fertilized; they were about 3.5 centimetres long and attached singly by a broad base to the sides of the cage in which the animals were confined.

LITERATURE. - Lacaze-Duthiers,"Observation d'un argonaute de la Mediterranee," Arch. zool. exper. x. (1902), p. 1892. Cephalopoda, by Jalta; Fauna and Flora des Golfes von Neapel, monographs issued by the Zoological Station of Naples. Bashford Dean, "Notes on Living Nautilus," Amer. Natur. xxxv. (1901). A. Willey, Contribution to the Natural History of the Pear l y Nautilus; A. Willey's Zoological Results, pt. vi. (1902). (J. T. C.)