46 
SHELL GALLERY. 
animal attaches itself most securely to anything it may seize ; they 
are employed in capturing food and in walking. Cephalopods walk 
in any direction head downwards, but can swim backwards only, 
The Common Octopus (Polypus vulgaris), resting. 
being propelled in that direction by the water which they discharge 
with force through the funnel out of their branchial cavity. They 
are divided, according to the number of their gills (which is either 
two or four), into JDibranchia and Tetra- 
branchia. Of the latter but one representa- 
tive now exists, viz., the Pearly Nautilus, all 
other living Cephalopods being provided with 
but two gills, placed one on each side of the 
body within the mantle, as may be seen in the 
wax model of Sepia officinalis (Case 207). The 
two-gilled section comprises forms with eight 
arms, as Argonauta and Octopus, and others 
with ten arms, viz., the Cuttlefishes (Sepia) 
(Fig. 43), the Squids (Loligo, Ommatostrephes, 
Sepiola, Chiroteuthis, etc.), and Spirula. The 
" shell " of the Paper-Nautilus, or Argonauta, 
is too well known to require any description. 
Unlike the shells of other Mollusca, it is not attached to the animal 
by a special muscle, but is held on to the body by two of the arms, 
Sepiola scandica 
(Natural size). British 
