MOLLUSCA: CEPHALOPODA. 421 



The mouth leads into a buccal cavity (fig. 225) containing 

 two powerful mandibles, working vertically, resembling the 

 beak of the parrot in shape, and either horny (as in the Cuttle- 

 fishes) or partially calcareous in composition (as in the 

 Nautilus). There is also a muscular tongue which appears to 

 be in part an organ of taste, whilst in part it is developed into 

 a lingual ribbon or " odontophore." The buccal cavity (fig. 

 225) conducts by an oesophagus into which salivary glands 

 usually pour their secretion to a stomach, from which an 

 intestine is continued, with a neural flexure, to open on the 

 ventral surface of the animal at the base of the funnel. A 

 large and well-developed liver is present. In many cases 

 there is also a special gland, called the "ink-bag," for the 

 secretion of an inky fluid, which the animal discharges into 

 the water, so as to enable it to escape when menaced or 

 pursued. The duct of the ink-bag opens at the base of the 

 funnel; but this apparatus is entirely wanting in the Tetra- 

 branchiate Cephalopods, where, in consequence of the presence 

 of an external shell, this means of defence is not needed. 



The kidneys (fig. 226, rr)are in the form of spongy cellular 

 organs developed upon the two posterior branches of the vena 

 cava. The circulatory organs consist of a systemic central 

 heart (fig. 226, c} which drives the aerated blood to all parts 

 of the body. The blood finds its way into the veins mostly 

 through the intervention of a system of capillaries, but also by 

 means of sinuses and lacunae amongst the tissues. The two 

 great trunks which carry the venous blood to the branchiae, 

 are further provided, in the Cuttle-fishes, with special con- 

 tractile dilatations, situated one at the base of each gill, and 

 known as the " branchial hearts " (e e). 



The respiratory organs are in the form of two (Cuttle-fishes) 

 or four (Nautilus) plume-like gills, placed symmetrically on the 

 sides of the body within the pallial sac. The gills (fig. 226, b b) 

 consist each of a central stem, bearing finely-divided lateral 

 vascular laminae ; and as they are not ciliated, the necessary 

 respiratory currents are maintained by the alternate contrac- 

 tions and expansions of the muscular walls of the mantle-sac. 

 In each expansion the water finds its way into the pallial 

 chamber by the opening between the rim of the mantle and 

 the neck ; and in each contraction it is expelled through the 

 tube of the funnel, which is so constructed as to allow of the 

 egress but to prevent the ingress of the water. 



The nervous system consists of the three normal pairs of 

 ganglia the cerebral, pedal, and parieto-splanchnic but these 

 are aggregated to form an oesophageal collar (fig. 225, ). 



