HISTORY OF THE EARTH 299 



(Figs. 350, 365, and 443). The snail-like mollusks 

 live on land, in fresh water, and in the sea. 

 (c) Chambered mollusks (Cephalopods). 



The cephalopods take rank as the highest of all 

 the mollusks. They have well-developed eyes, 

 and many are active, voracious animals, 

 i. Nautilus division. 



The animals of this division are now rare, 

 although many existed in earlier periods. The 

 shell is a long tube divided into chambers by a 

 series of cross partitions (Fig. 353). A small 

 tube or siphuncle extends back through all the 

 chambers to the apex of the shell. The earlier 

 shells were straight, but soon there appeared 

 curved (Fig. 354) and coiled forms (Fig. 355). 

 Spiral shells, however, are rare. Later in the 

 history of the cephalopods the partitions be- 

 came more or less folded into convolutions 

 (Fig. 391), which are shown on the inner sur- 

 face of the shell by angles and loops in the 

 lines (sutures) formed by the junction of the 

 partition with the shell. Eventually this fold- 

 ing of the sutures became extremely complicated 

 (Fig. 447). 

 ii. Cuttlefish division. 



The cuttlefish, octopus, and squid, although 

 common enough to-day, have left fewer fossils 

 than the last group. Their stout bodies, with 

 long, fleshy tentacles surrounding the head, 

 have no hard parts, except that some had a 

 cigar-shaped shell embedded in the body of 

 the animal (Fig. 429). 



(8) ARTHROPODS ("Jointed-leg" animals). 



Besides less familiar types, this group includes the 

 insects, spiders, centipedes, and crayfish, that is to 



