XII 



PHYLUM MOLLUSCA 



725 



laterial by means of which the eggs when deposited adhere 

 together in masses. A glandular mass of unknown function, 

 known as the accessory nidamental glands, 

 lies at the sides and around the oral 

 ends of the nidamental glands proper. 



ii. THE PEARLY NAUTILUS (Nautilus 

 pompilius}. 



spr 



The three living species of Nautilus, 

 of which N. pompilius is the best 

 known, are inhabitants of moderately 

 shallow water about the shores and 

 coral-reefs of the South Pacific, usually 

 creeping on the bottom at the depth of 

 a few fathoms, and probably never 

 coming voluntarily to the surface. The 

 body is enclosed in a calcareous, spirally- 

 coiled shell (Fig. 637), into which the 

 entire animal can be withdrawn for pro- 

 tection. The cavity of the shell is divided 

 by a system of septa into a series of 

 chambers, the last and largest of which, 

 opening widely on the exterior, alone 

 lodges the body of the animal. Between . 

 the animal and its shell there is a 

 direct organic connection through the 

 intermediation of a narrow tubular pro- 

 longation of the visceral region, which 

 perforates the entire series of the septa 

 to the apex of the spiral. This tube, 

 which is termed the siphuncle (si), has 

 its wall supported by scattered spicules of carbonate of lime; 

 but, in addition, as it passes through each septum, there is pro- 

 duced over it for some distance a shelly tube the septal neck 

 (s. ri) 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 protoconcfy 

 which has fallen off in the course of development. 



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 



B 



FIG. 630. Sepia. A, sperms, 

 highly magnified ; B, sperrna- 

 tophore ; sp. mass of sperms ; 

 spr. spring apparatus by which 

 the wall of the spermatophore 

 is ruptured. (From Vogt and 

 Jung.) 





