49 



trary, are increased in bulk commensurate with the larger funnel, and with the 

 more perfectly developed and capacious shell. To the presence of the latter 

 body is doubtless to be attributed the absence of the thick stratum of muscular 

 fibres which gives so peculiar a character to the mantle of the Dibranchiata ; 

 since in these we find that where the shell is advanced, as in Sepia, the muscular 

 fibres are wanting at the part of the mantle in which it is lodged. And, as in 

 addition to the difference in the structure of the mantle, the natatory organs of the 

 higher Cephalopoda have no analogues in the Pearly Nautilus, we must conclude 

 that in the active powers both of respiration and locomotion it is vastly inferior 

 to them. 



The digestive system, from the nature of its functions, and the similarity of 

 the materials on which it has to operate, maintains a more strict affinity, and, 

 except in a few particulars, scarcely varies in degree beyond those generic dif- 

 ferences which have already been observed in the liigher order of Cephalopods : 

 thus we have in the Pearly Nautilus the same general form and disposition of 

 jaws for dividing and comminuting the alimentary substances, but with a dif- 

 ference of texture adapted to the harder nature of the food which the submarine 

 habits and locomotive powers of this species probably confine it to. The pre- 

 paratory receptacle, or crop, is of a larger size than in the higher genera of 

 Cephalopods, among which, however. Octopus and Ocythoe in this respect come 

 nearest to Nautilus. The succeeding cavity, or laminated pancreatic bag, which 

 is the analogue of the spiral csecum of the Dibranchiata, presents a simplicity of 

 form, which, in conjunction with the slight development of the salivary organs, 

 constitutes the chief difference in the digestive system of Nautilus. In both orders 

 of Cephalopoda the intestine terminates in the same manner, and apparently for 

 the same intention, — viz. that the excrementory substances may be thrown within 

 the influence of the respiratory currents, and so be expelled by the funnel. But 

 in the disposition and increased length of the intestinal canal in Nautilus, we 

 may observe a deviation from the character so peculiar to the intestine of the 

 higher Cephalopods ; and both in this respect, and in the minutely subdivided 

 liver, there is an evident approximation to the Gasteropodous type of structure. 



It is, however, in the respiratory and circulating systems that dissection has 

 shown the greatest differences to subsist between Nautilus and the higher Cepha- 

 lopoda; but here also the differences depend on excess and defect of develop- 



H 



