Prof. Owen on the Structure of the Pearly Nautilus. 309 



and ' proboscides/ They have no true homology with the loco- 

 motive members of the Vertebrata, but are analogous to them, 

 inasmuch as they relate to the locomotive and prehensile faculties 

 of the animal. 



*' The eight arms of the Octopus commence by a hollow cone of 

 muscular fibres, attached by a truncated apex to the anterior part 

 of the cephalic cartilage. The fibres are for the most part oblique, 

 and interlace with one another in a close and compact manner 

 as the cone advances and expands to form the cavity containing 

 the mandibulate mouth, at the anterior extremity of which they 

 are continued forward, and separate into eight distinct portions 

 which form the arms. The development of the eight external arms 

 bears an inverse proportion to that of the body : they are longest 

 in the short round-bodied Octopi, and shortest in the lengthened 

 Calamaries and Cuttle-fishes, in which the two elongated retrac- 

 tile tentacles are superadded by way of compensation. These 

 latter organs are not continued from the muscular cone, which 

 corresponds with the great cephalic sheath supporting the exte- 

 rior tentacula in the Nautilus, but arise, like the internal labial 

 processes in that Cephalopod, close together from the cephalic 

 cartilage, internal to the origins of the ventral pair of arms. They 

 proceed at first outwards to a large membranous cavity situated 

 anterior to the eyes, and emerge between the third and fourth 

 arms on either side. 



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

 modified in the Dibranchiate Cephalopods, as in the Nautilus, for 

 the prehensile and tactile faculties ; but the structure is much 

 more complicated in the higher order, or Dibranchiata. On this 

 surface each arm supports a single or double series or more nu- 

 merous rows of acetabula or circular sucking-cups : in the elon- 

 gated 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 ten- 

 tacles 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, 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 peri- 

 phery, intercepting spaces containing the longitudinal fibres of the 

 arm, the whole being surrounded by two thin and distinct strata 

 of fibres, of which the external is longitudinal, and the internal 

 transverse. 



" The mechanical structure of the acetabulum may be favour- 

 ably studied in the Octopus, in which those organs arc of large size 



