134 



MOLLUSCA 



body. Amongst Gastropods it is not very unusual to find 

 the animal slipping forward in its shell as growth advances 

 and 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, 

 &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- 

 zoa. 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 



Fin. 97 Head and circum-oral pro- 

 cesses of the fore-foot of Onycho- 



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. 



FIG. 98. The calcareous internal shell of Sepia officinalis, the so-called cuttle- 

 bone, a, lateral expansion ; b, anterior cancellated region ; c, laminated 

 region, the laminre enclosing air. 



FIQ. 99. The horny internal shell or 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, b), 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) ; (<) the concrescence 



Fio. 100. Internal shells of Cephalopoda Siphono- 

 poda. A. Shell of Conotcuthis dvpiniana, d'Orb. 

 (from the Neocomian of France). B. Shell of 

 Sepia orbigniana, Fer. (Mediterranean). C. Shell 

 of Spirtilirostra Bellardii, d'Orb. (from the Mio- 

 cene of Turin). The specimen is cut so as to show 

 in section the chambered shell and the laminated 

 " guard " deposited upon its surface. D. Shell of 

 Spinila leevis, Gray (New Zealand). 



