CEPHALOPOnA. 



611 



222 



in the Dibrancliiate as in theTetrabranchiate Cephalopods for the pre- 

 hensile and tactile faculties ; but the structure is much more compli- 

 cated in the higher order. On this surface each arm supports a 

 single or double series or more numerous rows of acetabula or cir- 

 cular sucking cups : in the elongated pair of superadded tentacles of 

 the Decapods the suckers are limited to the expanded extremities, 

 where they are generally aggregated in more numerous and irre- 

 gular rows. These tentacles serve to seize a prey which may be 

 beyond the reach of the ordinary arms, and also act as anchors to 

 moor the Cephalopod in some safe harbour during the agitations of a 

 stormy sea. 



Each muscular arm is perforated near the centre of its axis for the 

 lodgement of its nerve and artery {^Jig. 222, a), which are surrounded 

 by a layer of cellular tissue (c) ; from 

 the dense outer sheath of this cellular 

 canal the transverse fibres of the arm 

 (cT) radiate to the periphery, intercept- 

 ing spaces containing the longitudinal «•- 

 fibres of the arm, the whole being sur- 

 rounded by two thin and distinct strata 

 of fibres, of which the external is 

 longitudinal, and the internal trans- 

 verse. 



The mechanical structure of the 

 acetabulum may be favourably studied 

 in the Octoptts, in which those organs 

 are of large size, and sessile. The 

 circumference of the disc of the sucker 

 is raised by a tumid margin (A) ; a 

 series of slender folds of membrane (/)> 

 covering corresponding fasciculi of mus- 

 cular fibres, converge from the circum- 

 ference towards the centre of the sucker, where a circular aperture 

 (ff) leads to a cavity which widens as it descends, and contains a soft 

 caruncle (i), rising from the bottom of the cavity like a piston of a 

 syringe. When the sucker is applied to any surface for the purpose 

 of adhesion, the piston, which previously filled the cavity, is retracted, 

 and a vacuum produced. 



The complex irritable mechanism of all these suckers is under 

 the most complete control of the predatory Cephalopod. My friend 

 Mr. Broderip, F.R.S.*, informs me, that he has attempted, with a 



♦ Author of valuable original Memoirs in the Transactions of the Zoolot^ical 

 Society ; of the admirable articles on zoology in the Penny CyclopEcdia ; 



BS 2 



Octopus. 



