DIBRANCHIATA. 137 



moveable or fixed ; the form of the body ; the pre- 

 sence or absence of fins at the sides ; the presence, 

 texture, and form of the shell ; the existence of a 

 web uniting the bases of the arms ; the arrange- 

 ment of the suckers ; the comparative length of 

 the arms ; the number of the suckers on each arm ; 

 the form, size, and hue of the colour spots, con- 

 stitute the points to be chiefly attended to in 

 distinguishing the species in this Class. 



Authorities. Siebold's " Comparative Ana- 

 tomy," with Dr. Burnet's Notes; Woodward's 

 " Manual of the Mollusca ; " and Gray's " Cata- 

 logue of the Cephalopoda in the British Museum." 



CEPHALOPODA. 



Molluscous animals without a foot, with a dis- 

 tinct head, crowned with numerous fleshy arms, 

 bearing sucking-disks ; enveloped in a sac-like 

 mantle, open in front, and sometimes enclosing in 

 its substance a rudimentary shell (the shell exter- 

 nal only in foreign species) ; having a rudimentary 

 brain, and a rudimentary skeleton of cartilage ; 

 breathing by gills, which are bathed by water 

 passing in beneath the mantle, and discharged 

 through a tube. 



Order I. DIBRANCHIATA. 



Animal with a naked body, a distinct head, 

 sessile eyes, horny jaws, eight arms ; a round or 

 oblong body formed for swimming ; two gills, and a 

 shell (when present), either horny or shelly, always 

 (in British species) concealed in the mantle. 



With two tentacles ; eyes moveable . Decapoda. 

 Without tentacles ; eyes fixed . . . Octopoda. 



