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 



