528 



CEPHALOPODA. 



meet the breeze. What their real function 

 may be is still to be determined ; hut the re- 

 moval of the erroneous impressions entertained 

 on this subject is the first step towards the at- 

 tainment of the truth. 



In our common Octopus, and most other 

 species of this genus, the feet are connected 

 together for some distance beyond the oral 

 sheath by membranes and muscles which form 

 a circular fin. This is their sole locomotive 

 organ when swimming; and by its powerful 

 contraction they are driven through the water 

 with a quick retrograde motion. In a species 

 which we have recently described* (Octopus 

 semipalmatus) the fin is extended only between 

 the four dorsal arms : a structure which must 

 occasion a characteristic difference in its mode 

 of swimming. 



The disposition of the muscles of the web- 

 like fin is as follows. There are two transverse 

 layers of fibres, the external arises from a white 

 line extending along the back-part of each 

 foot ; the internal from the sides of the same 

 feet between the attachments of the suckers. 

 These two strong muscular bands are con- 

 nected together as they pass from arm to arm 

 in the middle of the webs, and decussate one 

 another, so that the external become internal 

 and vice versa. Within these a thin layer of 

 longitudinal fibres extends to the free margin 

 of the webs ; and there is also a layer of ob- 

 lique longitudinal fibres externally, which arise 

 from the white line at the middle of each foot : 

 these fibres are shown at (k, k,Jig. 216,) the 

 transverse fibres at I, I. 



In the Cephalopods which possess the re- 

 tractile peduncles, the ordinary arms are gene- 

 rally short, and the first or dorsal pair are 

 commonly exceeded in length by the second ; 

 sometimes, indeed, as in the species of Loli- 

 gopsis, of which the figure is subjoined, 

 (fig. 209,) they go on progressively increasing 

 in length to the ventral or fourth pair, which 

 here resembles in its great development the 

 arms of the Octopods. The peduncles are 

 always longer, and more slender than the arms; 

 they exhibit these characters in the highest 

 degree in the genus Loligopsis, in which they 

 are frequently mutilated and lost; but the 

 examination of the nerve proceeding to the 

 mutilated stump sufficiently attests,in such 

 cases, the importance of the organ of which 

 this animal has been accidentally deprived. 

 The tentacles serve to seize a prey which may 

 be beyond the reach of the ordinary feet, and 

 also to act as anchors to moor the Cepha- 

 lopod in safety during the agitations of a 

 stormy sea. 



Each arm is perforated near the centre of its 

 axis for the lodgment of its nerve (a, fig. 214) 

 and artery (/>); and upon making a transverse 

 section of the arm, these are seen to be lodged 

 in a quadrangular or rhomboidal space (c) of a 

 light colour and apparently soft homogeneous 

 texture, but in which a few radiating fibres may 

 be discerned. This part is surrounded by four 



* S t ee P^ccedings of the Zoological Society for 

 March, 1836. 



Fig. 214. 



Section of an Arm and Suckers of a Poulp. 



groups of transverse striae forming as many seg- 

 ments of a circle, external to which there are 

 two thin circular strata of fibres. On making 

 a longitudinal section of the part the striated 

 segments are seen to consist of longitudinal 

 muscular fibres, and of the surrounding strata, 

 the fibres of the internal are longitudinal, and 

 those of the external transverse. It is easy to 

 conceive that, like the tongue in Mammalia, 

 the arms thus organized may be lengthened, 

 shortened, curved, and bent in all conceivable 

 directions. 



The acetabula or suckers with which the in- 

 ternal surface of the arms of the Dibranchiates 

 are provided, vary in relative position, in size, 

 in structure, and in mode of attachment, not 

 only in different species, but in different arms 

 in the same individual, and sometimes in diffe- 

 rent parts of the same arm. Thus in the pe- 

 dunclesof Loligopsis Veranii, the suckers on the 

 long cylindrical stem are sessile, while those on 

 the expanded extremity are supported on long 

 peduncles; and another remarkable instance 

 will presently be mentioned of suckers having 

 different structures for different functions in the 

 same arm. 



In the Dibranchiate genera which are charac- 

 terized by a soft thin skin, as the Argonaut, 

 Octopus, and Eledone, the suckers are soft 

 and unarmed ; in those genera which have a 

 hard and thick skin, as the Calamary and 

 Onychoteuthis, cuticular appendages are deve- 

 loped in the cavities of the suckers. 



An excellent description of the unarmed 

 acetabulum as it exists in the genus Octopus, is 

 given by Dr. Roget. 



The circumference of the disc is raised by a 

 soft and tumid margin (c, Jig. 214); a series 

 of long slender folds of membrane (f), cover- 

 ing corresponding fasciculi of muscular fibres, 

 converge from the circumference towards the 

 centre of the sucker, at a short distance from 

 which they leave a circular aperture (g) : this 

 opens into a cavity (h), which widens as it 

 descends, and contains a cone of soft substance 



