CEPHALOPODA. 461 



derived, however, from the corresponding cerebral ganglion. There are a 

 number of otoliths in Naiitilus, a single otolith in Dibranchiata. 



The mouth is provided with two jaws like a parrot's beak, calcified in 

 Nautilus , chitinoid in Dibranchiata, the larger ventral in position 1 . They 

 are borne upon the extremity of a prominent buccal cone or mass, which 

 is surrounded externally by an integumentary fold or buccal membrane 2 . 

 The cone contains a radula. The oesophagus is wide and dilated near the 

 apex of the visceral dome in Nautilus , narrow in Dibranchiata, and provided 

 in Octopoda with a crop-like dilatation. The stomach is large, and its walls 

 are muscular and like the gizzard of a bird in Nautilus and Octopus. The 

 pyloric is near the cardiac aperture, and leads into an intestine, slightly 

 coiled in Nautilus and Octopoda, which terminates in an anus situated in the 

 branchial cavity, between the bases of the gills in Nautilus, more anteriorly 

 on the posterior wall of the body in Dibranchiata. A caecum is appended 

 to the intestine near the anus in Nautilus, close to the pylorus in Di- 

 branchiata. In the latter it may be short and round, or longer and spirally 

 coiled. Its internal surface is generally produced into folds or processes. 

 A pair of glandular (? salivary) masses lie in the buccal cavity in Nautilus. 

 A pair of glands with a single duct lies externally to the buccal mass 

 behind the nervous centres in all Dibranchiata, and a second pair in front 

 of them in some Octopoda. The duct opens in front of the radula. There 

 is a liver surrounded by a firm membrane, composed of four lobes in 

 Nautilus, two lobes in Dibranchiata. There are two bile ducts, which 

 unite, and open into the intestine in Nautilus, into the intestinal end of the 

 caecum in Dibranchiata. The so-called pancreas exists in three forms : as 

 a yellowish coloured part of the liver opening into a dilatation of the bile 

 duct in Octopoda ; as glandular masses impacted in the thickened walls of 

 the bile duct in Loligo ; or as glandular caeca appended to the bile duct in 

 other Decapoda. A gland known as the ink-bag opens in all Dibranchiata, 

 either near (Sepia) or into the rectum, of which it is a diverticulum. It has 

 been found in specimens of extinct Belemnitidae. Its inky secretion a 

 pigment known as Sepia serves to hide the animal when attacked or 

 alarmed. 



The heart is placed posteriorly near the summit of the visceral dome. 

 It is square in Nautilus, and receives four branchial veins, one at each 

 corner. It is more or less pyriform in Decapoda, and transversely oval in 

 Octopoda, and in both it receives two branchial veins, which at their cardiac 

 ends are dilated, muscular, and contractile, thus forming two auricles. The 

 ventricle gives ofT a cephalic and abdominal aorta, at its opposite extremities 



1 The fossil beaks of Tetrabranchiata are known as Rhyncholites. 



2 This buccal membrane is lobed ; it sometimes bears suckers, and is supplied by nerves derived 

 from the brachial nerves, and therefore from the pedal ganglia. Viailleton regards it as representing 

 a series of arms nearly aborted with well-developed interbrachial membranes. See C. R. 100, 1885. 



