46 



SHELL GALLERY. 



animal attaches itself most securely to auything it may seize ; they ; 

 are employed in capturing food and in walking. Cephalopods walk 

 in any direction head downwards, but chiefly swim backwards, ^ 



Fig 40. 



Fig. 41. 



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 Dibranchia and Tetra- 

 hranchia. 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 the Paper-Nautilus {Argonauta) and 

 the Octopus {Polypus), and others with ten 

 arms, viz., the Cuttlefishes {Sepia) (Fig. 43), 

 the Squids {Loligo, Ommatostreplies, Sepiola, 

 ChiroteutMs, 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 



Sepiola scandica 

 (Natural size). British. 



