CLASSIFICATION 17 
in the Cuttlefishes, the latter half of the alimentary 
canal is bent underneath the body till its termination 
approximates the mouth. Then the visceral mass is 
twisted round laterally, so that the terminal portion 
of the alimentary canal, with the two gills and other 
paired organs on either side of it, are brought round 
by the right side till they lie nearly over the head, 
and the original left gill and accompanying organs 
become in position the right-hand ones, while the 
visceral nerve loop is twisted into a figure of 8. 
Next, the original left (now right) gill and its accom- 
panying organs tend to wither and eventually dis- 
appear. In some cases the secondary twist takes 
place in the reverse direction, or round by the left 
side, when a left-handed animal results, and the 
originally right gills, etc., are suppressed. 
In this way the animal becomes asymmetrical, 
organs on one side of the body not being matched 
by corresponding ones on the other. A further result 
of the secondary, rotary twist is that the shell, which 
was beginning to coil forwards over the animal’s head, 
or exogastrically, as it does in Nautilus, is swung 
round so that it coils backwards, or endogastrically. 
The shell secreted by the mantle covering this 
twisted visceral hump, of course, reflects its form 
in every respect. Essentially the resultant shell is 
a longer or shorter hollow cone. In some, such as 
the adult Limpet, it is a simple cone, but in by far 
the greater number it is an elongated cone, coiled 
2 
