CEPHALOPODA, 345 



In the Octopus velifer both the first and second pairs of arms sup- 

 port broad and thin membranous appendages at their extremities. In 

 the common Octopus the eight arms are connected together for some 

 distance beyond the head by membranes and muscles, which form a 

 circular fin ; this constitutes its sole locomotive organ when swimming, 

 and, by its powerful contraction, aided, however, by the ejection of the 

 currents from the funnel, the animal is propelled through the water 

 by a quick retrograde motion. In the Oct. semipalmatus the fin is 

 extended along the basal interspaces of only the four dorsal arms. 

 In the Cirroteuthis it extends between all the arms, and as far as 

 their attenuated extremities.* There are two layers of transverse 

 fibres in this web, the external of which arises from a white line 

 along the back part of the base of each arm, the internal from the 

 sides of the same arms between the attachments of the suckers. They 

 decussate one another as they pass from arm to arm in the middle of 

 the webs, and are included between two thin layers of radiating or 

 longitudinal fibres. 



The internal surface of the arms is that which is specially modified 

 in the Dibranchiate Cephalopods, as in the Nautilus, 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 irregular 

 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 Ce- 

 phalopod 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, which are surrounded by a layer of 

 cellular tissue ; from the dense outer sheath of this cellular canal the 

 transverse fibres of the arm radiate to the periphery, intercepting 

 spaces containing the longitudinal fibres of the arm, the whole being 

 surrounded by two thin and distinct strata of fibres, of which the ex- 

 ternal is longitudinal, and the internal transverse. 



The mechanical structure of the acetabulum may be favourably 

 studied in the Octopus, 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 series of slender folds of membrane, covering corre- 

 sponding fasciculi of muscular fibres, converge from the circumference 

 towards the centre of the sucker, where a circular aperture leads to a 



' Eschriclit, Acta Acad. Nat. Cur. vol. xviii. p. 627. 



