xii PHYLUM MOLLUSCA 755 



spicules of carbonate of lime ; but, in addition, as it passes through 

 each septum, there is produced over it for some distance a shelly 

 tube the septal neck (s. n.) continuous with the substance of the 

 septum. The apical or initial chamber presents a small scar, the 

 cicatrix, which may indicate the original presence of the larval 

 shell, or protoconch, which has fallen off in the course of develop- 

 ment. 



When the shell of the Nautilus is compared with that of Triton 

 some points of resemblance, together with important points of 

 difference, will be at once recognised. In both the growth of the 

 shell has taken place in such a way as to produce a gradual and 

 regular increase in the width of the internal cavity, from the apex 

 to the mouth, the result being a form of shell which, if it were 

 straightened out, would be a long cone. In both the growth has 

 not taken place in a straight line, but in a spiral, and a spiral of 

 so close a character that successive turns are in immediate contact 

 and their walls fused together. But in Nautilus all the turns of 

 the spiral are in the same plane ; the spiral, in other words, is a 

 flat one, as has already been found to be the case in certain of the 

 Gastropoda (p. 718), whereas in Triton the spiral is an elongated 

 helix : in other words, the spiral of Nautilus is that of a watch- 

 spring, that of Triton that of a corkscrew. The possession by 

 Nautilus of the series of septa marking the position which the 

 animal has occupied at successive stages in its growth is another 

 striking difference. Moreover the relations of the soft parts of 

 the shell are radically different in the two cases. In Triton the 

 body is attached to the shell by the columellar muscle ; in 

 Nautilus the main organic connection is by means of the siphuncle ; 

 for, though it is chiefly through the pressure exerted by two great 

 lateral masses of muscle (Fig. 684, mus.) that the Nautilus retains 

 its hold of the shell, the muscular fibres are not attached to the 

 latter in the same intimate way as those of the columellar muscle 

 of Triton, but are inserted into a horny cuticular membrane inter- 

 vening between the muscle and the shell. Again, while the curva- 

 ture of the body of Triton with the enclosing shell is towards the 

 ventral side (endogastric), in Nautilus it is towards the dorsal side 

 (exogastric). 



When the animal is removed from the shell it is found to possess 

 two regions, a distinct and relatively large, obtusely conical head 

 bearing eyes and a system of tentacles, and a rounded sac-like 

 trunk. Both head (or cephalopodium) and trunk are very slightly 

 compressed, the direction of the compression being, as in Sepia, 

 from the antero-dorsal towards the postero- ventral side, and 

 they are almost completely bilaterally symmetrical, only a very 

 slight disturbance of the symmetry being discernible. The mouth, 

 situated at the free extremity, is provided with a pair of relatively 

 enormous, partly calcified jaws (Fig. 684), Surrounding the mouth 



