Io6 A NATURAL HISTORY OF THE SEAS 



gills, and the Dibranchia, having but one pair of gills and 

 embracing all the other species — Octopuses, Squids,Cuttle- 

 fish and Argonaut. 



The Tetrabranchia are to-day represented by only four 

 closely related species of Nautilus, all from the Indo- 

 Pacific. The animals are of interest since they are the sole 

 survivors of a race that once swarmed in all salt waters 

 and are now known only by their fossilised shells. 



It is now generally agreed that all Cephalopods were 

 originally shell-bearing molluscs, the shell as seen in the 

 Octopus and Cuttlefish having become overgrown by the 

 animal's body wall or reduced to a mere vestige. The 

 earliest Cephalopods lived in small conical shells, and as 

 the animal grew, it shut off behind it that portion of the shell 

 which it no longer needed for retreat. Thus the shell 

 became eventually divided into a series of chambers, 

 which, judging by the living Nautilus, were filled with 

 gas to give it buoyancy. Some of these early conical 

 shells reached a length of 15 feet. In later forms they 

 were coiled, at first loosely, later tightly, into a spiral 

 form, more convenient no doubt for rapid transit. The 

 tightly-coiled forms survived much longer than those 

 with the cumbrous conical shells. Familiar to many 

 are the immense Ammonite Beds of the West Country, 

 which in some parts are literally solid with shells, many 

 of which reach two feet or more in diameter. 



The shells of the living Ammonite or Nautilus are common 

 enough as ornaments or museum exhibits, but few have 

 seen the living animal. The animal is abundant in the 

 Southern Philippines, where it is a common by-product 

 of the fishermen's traps and is prized as food by the natives. 



