XII 



PHYLUM MOLLUSCA 



741 



the body of the Nautilus shifts forwards at intervals into a 

 newly formed chamber, and a new septum is formed closing the 

 latter off from the cavity last occupied. 

 Of existing Dibranehiata, Spirula alone 

 has a shell (Fig. 651) comparable to that 

 of Nautilus. The shell of Spirula is of 

 spiral form, the turns of the spiral, how- 

 ever, not being in close contact. In- 

 ternally it is divided into chambers by 

 a series of septa, and these are perforated 

 by a siphuncle. But the initial chamber 

 (protoconch) instead of being, like the 

 initial chamber in Nautilus, similar to 

 the others though smaller, is dilated into 

 a spherical shape, constricted off from the 

 succeeding chamber, and has passing 

 through it a tube the prosiplwn not 

 continuous with the siphuncle. Again, 

 as will be seen by comparing Figs. 638 

 and 652, the relation of the soft parts to 

 the shell is the reverse of what obtains 

 in Nautilus, the shell of Spirula curving 

 backwards, that of Nautilus forwards. 

 Moreover the shell of Spirula (Fig. 652) 

 is an internal structure, being almost 

 completely covered by the mantle. 



The shell of the extinct Ammonites (Fig. 653), which are usually 

 referred to the Tetrabranchiata, resembles that of the Nautilus in 

 many respects, being a chambered spiral shell with a large terminal 

 chamber, and with a siphuncle. The chief external difference is 



in the form of the sutures, or lines 

 of union of the edges of the septa 

 with the side wall of the shell ; these 

 are more or less complexly lobed, in- 

 stead of being entire as in Nautilus. 

 But in one important respect the 

 shell of an Ammonite differs from 

 that of Nautilus and approaches that 

 of the dibranchiate Spirula. At the 

 apex of the spiral is an initial cham- 

 ber or protoconch, which is dilated 

 and separated from the first of the 

 ordinary chambers by a constriction, 

 and has passing into it a prosiphon not continuous with the 

 siphuncle. The Ammonite was also characterised by the posses- 

 sion of a structure sometimes horny, sometimes calcareous, called 

 the aptychus, not represented in any existing form. The aptychus, 



FIG. 652. Spirula peronii, 



lateral view ; d, terminal 

 sucker; /. funnel; s. s.i s.2 

 projecting portions of the 

 shell, the internal part of 

 which is indicated by dotted 

 lines. (From Cooke.) 



FIG. 653. An Ammonite (Ceratites 

 nodosus.) 



