SQUIDS, CUTTLE-FISH, AND THEIR ALLIES 161 
by having ten arms, the additional pair being 
of unusual length and only having suckers 
at their extremities. 
In some instances the arms are completely 
retractile, in others but partly so, while, yet 
again, they may be non-retractile. They serve 
their owners as prehensile organs by means 
of which they can affix themselves to objects, 
and also as lasso-like weapons for seizing their 
prey. 
All of the creatures possess an internal 
shell, that of the cuttle-fish, known com¬ 
mercially as “ cuttle-bone,” being of a bony 
nature, broad and flat, and of considerable 
size in relation to the body of the owner; 
but in the squids the structure, called the 
“ pen,” is horny and very slender. 
The latter, several of which may be present 
in old individuals arranged the one behind 
the other, resembles a quill pen in form, the 
stem being enlarged at one end and forming 
an oval-shaped and flat expansion that is 
vaned like a feather. As the creatures are 
in the habit of ejecting a dark-coloured fluid 
from their siphon for the purpose of rendering 
opaque the water in which they are swimming, 
thereby enabling them to escape from their 
enemies, they are frequently spoken of as “pen- 
and-ink ” fish. 
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