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 XXII.) 
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, 
