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as to have rendered it necessary, in the description, to refer to a Gasteropodous 

 genus in illustration of it. The essential difference, however, as has already been 

 shown, consists in the simpler condition of the central mass; — the source of 

 volition being thus in harmony with the diminished energies of the muscular and 

 the contracted sphere of the sensitive system. 



The differences in the distribution of the principal nerves are not less impor- 

 tant in a physiological point of view. In the Cephalopods, whose shells are 

 rudimentary and internal, and whose bodies are enveloped in a naked, and as 

 we must suppose sensible mantle, the nerves which supply that part radiate 

 from a ganglion, which, as in the posterior roots of the spinal nerves in the 

 Vertehrata, is interposed on the chord which brings them in communication with 

 the central mass. In Nautilus, on the contrary, whose body is incased in an in- 

 sensible calcareous covering, the analogous nerves are wholly expended on the 

 largely developed muscles which attach the shell to the body ; and these nerves, 

 like the motor filaments of the spinal nerves, pass into the muscles directly from 

 the brain without the interposition of any such ganglion. 



The inferiority of the more intellectual senses, sight and hearing, is in corre- 

 spondence with the simphcity of the brain. If, as I believe, a distinct organ for 

 the latter sense is altogether wanting, the Pearly Nautilus exhibits in this respect 

 an obvious approximation to the inferior Mollusks ; and in receding from the higher 

 Cephalopods in the structure of the eye, it inclines more directly towards the 

 Gasteropods, numerous genera of which, and especially the Pectinihranchiata of 

 Cuvier, present examples of this organ analogous in simphcity of structure, and 

 in a pedicellate mode of support and attachment to the head. As the Pearly 

 Nautilus, hke the latter group of Mollusks, is also attached to a heavy shell, 

 and participates with them in the deprivation of the locomotive instruments of 

 the Cephalopods, we may thence deduce the more immediate principle of their 

 reciprocal inferiority with respect to the visual organ ; for what would it avail 

 an animal to discern distant objects, which could neither overtake them if ne- 

 cessary for food, nor avoid them if inimical to its existence ? 



As the spheres of vision and of action, however, are thus limited, the power 

 of taking cognizance of proximate objects is proportionately augmented, and 

 the organs of the simpler sense of touch are more amply developed. In the nu- 



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