THE MOLLUSCA 93 



footed molluscs ; the Gastropoda, or stomach-footed molluscs, 

 the univalves ; the Pelecypoda, or axe-footed molluscs, the bivalves ; 

 and the Scaphopoda, or boat-footed molluscs, the tooth-shells. 

 To these may be added a fifth order, the Pteropoda, or wing-footed 

 molluscs, whose exact position is still somewhat doubtful, some 

 authorities regarding them as a subdivision of the Gastropods, 

 while others consider them as forming a separate order. 



In the first order, the Cephalopoda, the feet (more popularly 

 spoken of as " arms ") are arranged symmetrically round the 

 head or mouth ; and the order includes the Octopus, the Argonaut 

 or Paper Nautilus, the Cuttlefish, the beautiful Pearly Nautilus, 

 and the fossil Belemnites and Ammonites. They are all marine, 

 fairly active, and carnivorous. The Cuttlefish, Octopus, and Argo- 

 naut possess only two plume-like gills or breathing organs, while 

 the Pearly Nautilus, sole survivor of a once very numerous order, 

 possesses four. x The Nautilus and the Argonaut are the only living 

 representatives of the Cephalopoda that have an external shell, the 

 other members having an internal " shell " hidden in the soft part 

 of their body (Sepia, Loligo). Those members of the Cephalo- 

 poda possessing eight arms are termed the Octopoda, while those 

 with ten arms comprise the Decapoda. 



The Argonaut, or Paper Nautilus, was known to the ancients, 

 and described by Aristotle as floating on the surface of the sea in 

 calm weather, holding up its two sail-shaped arms to catch the 

 breeze, and sailing along like a miniature ship upon the bosom 

 of the ocean. This pretty but absolutely false idea was generally 

 accepted, and handed down through the centuries as an estab- 

 lished fact. Artists and poets alike helped to popularise this 

 myth, the former by depicting the animal with six arms extended 

 over the sides of its frail shell to act as oars, and with the two 

 other arms expanded and upraised to act as sails ; while the 

 poets would have us credit it with giving man his first lesson in 

 navigation. Pope bids us 



" Learn of the little Nautilus to sail, 

 Spread the thin oar, and catch the driving gale." 



The most charming poetic description of the mythic habits of 



1 Those Cephalopods having two gills form the order Dibranckiala, and those 

 with tour gills the Tetrabranchiata. 



