38 THE LIFE OF THE MOLLUSCA 



the majority of Nautiloids and Ammonites it is more 

 or less coiled discoidally, and generally the coil is in 

 a forward direction over the animal's back, or " exo- 

 gastric." A few, however, are coiled in the reverse 

 direction, or are " endogastric" ; while in some cases 

 the coil is produced into a helicoid, or even a turricu- 

 lated spire. In yet other instances the shell may be 

 coiled in the young state, and become more or less 

 uncoiled (Scaphites, Crioceras, Hamites, etc.) or even 

 straight (Baculites) in the adult (Plate XXIL). 



The striking feature of the Cephalopod shell, how- 

 ever, lies in its internal structure (Plate I.). As the 

 animal grows, it builds on to the open end of the 

 shell to obtain increased accommodation, just as the 

 Gastropod does; but since it grows in girth more than 

 in length, it has, in order to obtain the requisite space, 

 to shift bodily forward in its shell. This takes place 

 gradually by the forward growth of the shell muscles 

 on either side and the intervening pallial attach- 

 ments (" annulus ") till an unoccupied space is thus 

 left behind, which is then partitioned off by a shelly 

 wall (" septum ") ; in this way the series of chambers, so 

 familiar in sections of the Nautilus shell, are formed, 

 each marking the completion of a period of growth. 

 The septa are, however, not entire, a perforation in 

 each is connected with that in its neighbour by a 

 pipe, in part calcareous, in part chitinous, the whole 

 forming a continuous tube which passes from the 

 outermost chamber, or that occupied by the animal, 



