XIII 



CEPHALOPODA— DIBRANCHI ATA 



381 



system (p. 206), the eye (p. 182), the radula (p. 236), and the 

 ink-sac (p. 241). 



One of the most characteristic features of the Dibranchiata 

 are the acetabular or suckers, with which the 

 arms are furnished. They are usually disposed 

 on the sessile arms in rows (of which there are 

 four in most Sepia, two in Octopus, and one in 

 Uledone}, Siudhecome more numerous and smaller 

 at the tip of the arm. They are massed together 

 in large numbers of unequal size on the 'clubs' 

 in the Decapoda, particularly in Loligo. In most 

 Octopoda their base is flush with the surface of 

 the arm, but in Decapoda the acetabula are ped- 

 unculate, or raised on short stalks. In Octopoda 

 again, the acetabula are fleshy throughout, but 

 in the Decapoda they are strengthened by a 

 corneous rim with a smooth or denticulate edge 

 QOmmastrephes, Architeuthis). Many of the 

 acetabula on the tentacular and sometimes on 

 the sessile arms of the Onychoteuthidae enclose 

 a powerful hook, which is retractile like the 

 claws of a cat. 



In mechanical structure the acetabula consist 

 of a disc with a slightly swollen margin, from which a series of 

 muscular folds converge towards the centre of the 

 disc, where a round aperture leads to a gradually 

 widening cavity. Within this cavity is a sort of 

 button, the caruncle, which can be elevated or 

 depressed like the piston of a syringe ; thus when 

 the sucker is applied the piston is Avithdrawn and 

 a vacuum created (Owen). 



In many Octopoda the arms are connected by 

 a web (the umbrella'), which sometimes extends up 

 the greater part of the arms (CirrJioteuthis, some 

 Eledone), at others occurs only at the base. The 

 use of the umbrella is perhaps to assist in loco- 

 motion, by alternate contraction and expansion. 

 A cartilaginous skeleton is well developed, especially in the 

 Decapoda. In Sepia a cephalic cartilage forms a complete ring 

 round the oesophagus, the eyes being situated in lateral prolonga- 



FiG. 239. — « Club' 

 of Loligo vul- 

 garis L., show- 

 ing the crowded 

 pedunculate ace- 

 tabula, X \. 



Fig. 240. — One of 

 the suckers 



oi Architeuthis 

 dux Stp., show- 

 ing the denti- 

 culate margin 

 and corneous^ 

 ring ; p, ped- 

 uncle. 



