Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/699

CEPHALOPODA..] MOLLUSCA 671 body. Amongst Gastropods it is not very unusual to find the animal slipping forward in its shell as growth advances und leaving an unoccupied chamber in the apex of the shell. This may indeed become shut off from the occupied cavity by a transverse septum, and a series of such septa may be formed (fig. 42), but in no Gastropod are these apical chambers known to contain a gas during the life of the animal in whose shell they occur. A further peculiarity of the Nautilus shell and of that of the allied extinct Am monites, Scaphites, Orthoceras, <i c., and of the living Spirula, is that the series of deserted air-chambers are traversed by a cord -like pedicle extending from the centro-dorsal area of the visceral hump to the small est and first-formed chamber of the series. No structure com parable to this siphuncular pedicle is known in any other Mollusca. Its closest repre sentative is found in the so- called " contractile cord " of the remarkable form Rhabdo- pleura, referred according to present knowledge to the Poly- xoa. There appears to be no doubt that the deserted cham bers of the Nautilus shell con tain in the healthy living animal a gas which serves to lessen the specific gravity of the whole organism. The gas is said to be of the same com position as the atmosphere, with a larger proportion of nitrogen. With regard to its origin we have only conjec tures. Each septum shutting off an air-containing chamber is formed during a period of quiescence, probably after the reproductive act, when the vis ceral mass of the Nautilus may be slightly shrunk, and gas is secreted from the dorsal inte gument so as to fill up the space previously occupied by the animal. A certain stage is reached in the growth of the animal when no new cham bers are formed. The whole process of the loosening of the animal in its chamber and of its slipping forward when a new septum is formed, as well as the mode in which the air-chambers may be used as a hydrostatic apparatus, and the relation to this use, if any, of the siphuncular pedicle, is involved in obscurity, and is the subject of much in genious speculation. In connexion with the secretion of gas by the animal, besides the parallel cases ranging from the Protozoon Arcella to the Physoclistic Fishes, from the Hydroid Siphonophora to the insect-larva Corethra, we have the identical phenomenon observed in the closely- allied Sepia when recently hatched. Here, in the pores of the internal rudimentary shell, gas is observable, which has necessarily been liberated by the tissues which secrete ftl a 3fir Fio. 97. Head and circum-oral pro cesses of the fore-foot of Onycho- teuthis (from Owen), a, neck ; b, eye ; c, the eight short arms ; <7, long prehensile arms, the clavate extre mities of which are provided with suckers at e, and with a double row of hooks beyond at /. The tempo rary conjunction of the arms by means, of the suckers enables them to act in combination. the shell, and not derived from any external source (Huxley). The coiled shell of Nautilus, and by analogy that of the Ammonites, is peculiar in its relation to the body of the animal, inasmuch as the curvature of the coil proceeding Fig. 98. Fig. 99. Fio. 98. The calcareous internal shell of Sepia offic.inalis, the so-called cuttle- bone, a, lateral expansion ; b, anterior cancellated region ; c, laminated region, the lamin:e enclosing air. FIG. 99. The horny internal shell ov gladius or pen of Loligo. from the centro-dorsal area is towards the head or forward, instead of away from the head and backwards as in other discoid coiled shells such as Planorbis ; the coil is in fact absolutely reversed in the two cases. Amongst the extinct allies of the Nauti lus (Tetrabranch- iata) we find shells of a variety of shapes, open coils such as Scaphites, leading on to per fectly cylindrical shells with chamber succeeding cham ber in a straight line (Orthoceras), whence again we may pass to the cork-screw spires formed by the shell of Turrilites. Whilst the Tetra- branchiata, so far as we can recognize their remains, are characterized by these large chambered shells, which, as in Nautilus, were with the exception of some narrow-mouthed forms such as Gomphoceras but very partially covered by reflexions of the mantle-skirt (fig. 89, 6), the Dibranchiata present an interesting series of gradations, in which we trace (a) the diminution in relative size of the chambered shell ; (b) its complete investiture by reflected folds of the mantle (Spirula, fig. 100, D) ; (c) the concrescence Fio. 100. Internal shells of Cephalopoda Siphono- poda. A. Shell of Conottitthis dupiniana, d Orb. (from the Neocomian of France). B. Shell of Sepia orbigniana, Fer. (Mediterranean). C. Shell of Spirvlirostra Bdlardii, d Orb. (from the Mio cene of Turin). The specimen is out so as to show in section the chambered shell and the laminated " guard " deposited upon its surface. D. Shell of

Spirula Isevis, Gray (New Zealand).