100 STRUCTURE AND RANK. 



throughout their animal and vital organs. The muscular system 

 forms a large proportion of the body, with various arrangements 

 and complications unknown in the lower Mollusca. The 

 strong hooked jaws work vertically as in the vertebrate animals, 

 and are accompanied by a thick muscular tongue covered with 

 spines. The respiratory apparatus is enormously developed ; the 

 nervous centres in the head receive a marked increase in bulk, 

 constituting them a tnie brain, which is enclosed in a cartilage, 

 and gives off nervous cords on each side to the organs of vision ; 

 while these, again, arc of an exceedingly complex structure, like 

 those of the vertebrate animals. In the higher Cephalopods 

 there are distinct organs for hearing, and the heart is three- 

 chambered ; while in several of the groups, the external conform- 

 ation of the animals, and many of their habits, bear a strong 

 resemblance to what is observed amongst the superior class of 

 fishes. 



Professor Owen separates these animals into two great divi- 

 sions, according to the number of their branchiae or gills, and 

 christens them respectively the Dibranchiata and the Tetrar- 

 branchiata the two-gilled and the four-gilled Cephalopods. The 

 two-gilled division takes the highest rank, and includes all the 

 naked Cephalopods, together with the little Argonaut, or Paper 



Nautilus 



" The sea-born sailor of his shell canoe," 



which has ever been such a pet with the poets, aud such a 

 puzzle to the philosophers. The four-gilled branch of the family 

 is represented in our existing seas only by the Pearly Nautilus 

 (Nautilus pompilius), and one other closely-related species these 

 two being now the sole survivors of that multitudinous host of 

 Orthocerarites, Ammonites, Hamites, Baculites, Turrulites, and 

 other allied forms, which swarmed in the ancient ocean, and at 

 one time held the undisputed empire of the seas. 



It will be but dealing with our subject after the approved 

 " historical " fashion, if, before entering more into detail on the 

 Cephalophods of our own time, we now glance for a brief space 

 at those ancient representatives of the race whose former ex- 

 istence is known to us from the fossilized remains of their shells 

 alone. 



The Cephalopods were amongst the most numerous and 

 powerful inhabitants of the ancient ocean- It may truly be 



