156 ANIMAL KINGDOM. 



The mouth of the Cephalopods has generally a pair of horny- 

 mandibles, like the beak of a hawk in form ; and these fossil beaks 

 have been called Bhyncholites. 



These chambered shells containing Cephalopods were once ex- 

 tremely numerous ; but only half a dozen living species are known, 

 and these are of the genus Nautilus. Modern Cephalopods are almost 

 exclusively naked species, having an internal shell, if any. In a few 

 species, as in the genus Spirula, the internal shell is chambered and 

 coiled (the coils not touching) ; but in the rest it is straight, and 

 serves only to stiffen the soft body. In the Cuttle-fish it is spongy- 

 calcareous. In the Squid, or Calamary, — a more slender animal, 

 requiring some flexibility for its movements, — it is horny, and is 

 called the pen (p, fig. 169). In some cases it has a small conical 

 cavity at the lower end. In the Belemnite, a group of fossil species, 

 it was stout, cylindrical, and calcareous, with a deep conical cavity, 

 and on one side the margin was prolonged into a thin blade 

 (figs. 702, 703). 



The Cephalopods are divided into — 



(1.) Dibranchiates, having two gills or branchise, as in the Octopus, Cuttle-fish, 

 Squid, Belemnite, Spirula, Argonaut, — including, therefore, all existing naked 

 Cephalopods, besides the Argonaut (the Paper-nautilus), whose shell is peculiar 

 in not being chambered. 



(2.) Tetrahranchiates, having four gills, as the name implies; as in the Nautilus 

 and the chambered shells of ancient time. The Orthoceras (figs. 257, 313, 314) 

 was a straight form with plane septa or partitions : the name is from the Greek 

 for straight horn. The Nautilus is a coiled form with plane partitions, and the 

 siphuncle central or subventral. The Ammonite group (figs. 700, 701, 765, 766) 

 contains coiled straight forms with the partitions plaited or zigzag (fig. 765 b) 

 at the margin, and a dorsal siphuncle. 



2. Cephalates. — The Cephalates are divided into two groups : — 

 (1.) The Gasteropods, the group containing the Univalve shells, as 

 well as some related species without shells, — the animals of which 

 crawl on a flat spreading fleshy piece called the foot (fig. 166) ; and 

 hence the name, from the Greek, implying that they use the venter 

 or under surface for a foot. 



(2.) The Pteropods, which swim by means of wing-like appendages 

 to the head (fig. 167), — to which the name refers, meaning wing- 

 footed. 



The Gasteropods, which embrace nearly all the cephalate Mollusks, have usually 

 a spiral shell, as in the common Snail, Buccinum, Turbo, etc. The mantle 

 of the animal is sometimes prolonged into a tube or siphon in front, to convey 

 water to the gills ; and in this case the shell has a canal at the beak for the pass- 

 age of the siphon. The modern marine univalves without a beak, the Natica 



