CEPHALOPODA — DIBRANCIIIATA 



381 



system (p. 206), the eye (p. 182), the radula (p. 23 6), and the 

 ink-sac (p. 241). 



One of the most characteristic featiires of the DiViranehiata 

 are the acetahula, or suckers, with which the 

 arms are furnislied. They are usually disposed 

 on the sessile arms in rows (of whicli there are 

 four in most Scjna, two in Octojras, and one in 

 Eledone), and become more numerous and smallei- 

 at the tip of the arm. They are massed together 

 in large numbers of unequal size on the ' clubs ' 

 in the Decapoda, particulaily in Loligo. In most 

 Octopoda their base is ilusli with the surface of 

 the arm, but in Decapoda the ace tabula are ped- 

 unculate, or raised on short stalks. In Octopoda 

 again, the acetabula are fleshy throughout, Init 

 in the Decapoda they are strengthened by a 

 corneous rim with a smooth or denticulate edge 

 {Ominastrephes, Arcliiteuthis). Many of the 

 acetabula on the tentacvdar and sometimes on fxg. 239. — 'Club' 



of Lolijo vul- 

 garis L. , sliow- 

 ing the crowded 

 pedunculate ace- 

 tabula. X I. 



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 whicli 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 t)f 

 button, the cdrwiicle, which can be elevated or 

 depressed like the piston of a syringe ; thus when 

 tlie sucker is applied the piston is withdrawn and 

 a vacuum created (Owen). 



In many Octopoda the arms are connected by 

 ot Architenthis ,^ ^ygjj u\^q umhreUct), wliicli sometimes extends up 

 ing the denti- the greater part 01 the arms \U%rriioteuili%s, some 

 cuiate margin j^Yg^^y^i^) .^t others occiuTS Only at the base. Tlie 



and corneous '^ *' . . , 



ring; p, ped- use of thc Umbrella IS perhaps to assist in loco- 

 uncle, motion, l>y alternate contraction and expansion.. 

 A cartilaginous skeleton is well developed, especially in the 

 Decapoda. In Se-pia a cephalic cartilage forms a complete ring 

 round the oesophagus, the eyes being situated in latei-al jjrolonga- 



FiG. 240. — One 

 of the suckers 



