CEPHALOPODA. 347 



organs ; we find it equally regulating the development of the peculiar 

 prehensile instruments of the Cephalopodic class. 



At first very numerous, comparatively small and feeble, essen- 

 tially alike, the cephalic tentacles of the Nautilus strikingly illus-* 

 trate the law of vegetative or irrelative repetition. Their primary 

 import is however plainly indicated by the direct derivation of their 

 central nerve from the cephalic ganglion ; and they present the 

 same complex plan of arrangement of their muscular fibres which 

 characterises the arms and tentacles of the dibranchiate Cepha- 

 lopods. The prehensile surface of the tentacula of the Nautilus is 

 made adhesive after the type of the simple laminated sucker of the 

 Remora; the median longitudinal impression which partially divides 

 the lamella, may represent the complete interspace which separates 

 into two series, in the arms of most of the Dibranchiates, the more 

 complex suctorial appendages which are developed on their internal 

 surface : but at all events, the reduction of these arms in number, 

 their augmentation in size, and perfection as prehensile instruments 

 by the superadded complications, are phenomena which ordinarily 

 attend the march of development. The order of this progress 

 would be anomalously reversed if the tentacles of the Nautilus repre- 

 sented, as M. Valenciennes supposes, the caruncles of the acetabula, 

 and the hollow processes of the oral sheath the cavities of those 

 appendages of the arms of the Dibranchiata. According to the 

 French Malacologist, the anterior circumference of the head or oral 

 sheath in the Nautilus represents four of the eight arms develoj^ed 

 therefrom in the Dibranchiata, and the two dorsal arms consist each 

 of two enormous acetabula, whose cavities are deepened into tubes, and 

 whose caruncles are produced into tentacula as highly organised in regard 

 to their nerves and muscles, as are the acetabuliferous arms themselves 

 in the higher orders. The four other arms of the Octopus are repre- 

 sented, according to M. Valenciennes, by the four groups of tentacula 

 which are included within the oral sheath in the Nautilus. Such is 

 not, however, the place of origin of any of the eight arms in the Di- 

 branchiata ; nor is it conformable with the general law of develoji- 

 ment, that a prehensile organ consisting of two large and highly 

 complicated acetabula in a low oi'ganised Cephalopod should support 

 two hundred smaller and more simple suckers in the higher organised 

 species. 



Fasciculi of muscular fibres are continued from the ventral pair of 

 feet and the back part of the cranium to the muscular partition which 

 divides longitudinally the branchial cavity : other fibres descend to 

 join the muscular tunic enveloping the liver and oesophagus. In the 

 cuttle-fishes and calamaries the branchial septum is not developed. 



