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, Avith 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 



^/e^owe), and become 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^ ArcMteuthis). 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 wl" „.x 

 the sucker is applied the piston is withdrawn and 

 a vacuum created (Owen). 



^^ThT" suckers In many Octopoda the arms are connected by 

 oi ArcMteuthis ^ web (the wm^reZ/a), which sometimes extends up 



dltX Stp., show- , ^ , c l^ /-/Y'T 7- 



ing the denti- the greater part 01 the arms ( Ctrrhoteuthis, some 



cuiate margin ^ledone), at others occurs only at the base. The 

 and corneous „i^ in- ■, j - - ^ 



ring; p, ped- Hse 01 the Umbrella is perhaps to assist in loco- 

 uncle, 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 §. 



