160 AFFINITIES AND GEOGRAPHICAL 



requisite change in the mode of respiration namely, from 

 branchiae, the apparatus necessary in aquatic life, to a vas- 

 cular air-sac, the first form of lungs the proper breathing 

 organ of terrestrial animals. 



In the peculiarly destructive Cephalopoda, we recognise 

 the highest organization of which the molluscan form appears 

 capable ; it includes the orthoceratites, ammonites, belemnites, 

 etc. of the rock systems, and the nautilus and cuttlefish of the 

 present era. Their descent is probably from the carnivorous 

 families of the pteropoda ; for " the nucleus of their shells," 

 says the naturalist last quoted, " is a spiral univalve, similar 

 in form to the undeveloped shells above alluded to [those of 

 the embryo gasteropods] ; and it is yet to be seen whether 

 all cephalopoda do not commence their existence under a 

 spiral-shelled f>teropodous form." It has also been remarked, 

 that " the shells of two species [of pteropoda] afford indica- 

 tions of a transition towards the cephalopoda ; one resembling 

 in its straight conical form the belemnite and many other ex- 

 tinct genera of that class, and the other having a partially 

 formed chamber at the lower closed extremity ; and similar 

 evidence is afforded by their internal structure." ( 75 ) This ge- 

 nealogy, if it shall be affirmed, will afford an important 

 illustration of the geological history, because it will show that 

 cephalopoda might be expected to make their appearance as 

 early in the rock series as any other mollusks possessing parts 

 equally fitted to commemorate their existence. These ani- 

 mals are to be supposed as an ultimate form, reached, not 

 through the medium of all the lower molluscan orders, but 

 only of one, and with respect to that one, it so happens that, 

 though possessing hard parts of such delicacy as to have little 

 chance of preservation, relics of it have been discovered as far 

 down as any cephalopodous remains.( 76 ) This contemporaneity 

 of the cephalopoda with the gasteropods and brachiopods, it 

 may be remarked, would be in harmony with what we know 

 of the economy of nature with respect to the destructive ani- 

 mals. They seem to bear a relation to those upon which 

 they are destined to prey, and to be a necessary accompani- 

 ment to them. Hence they would require to be upon a diffe- 



