MOLLUSCA. 279 



eight or more tentacles or arms. The body is symmetri- 

 cal, and wrapped in a muscular mantle. 



The nervous system is more concentrated than in other 

 Invertebrates ; the cerebral ganglia are partly enclosed in 

 a cartilaginous cranium. All the five senses are present. 

 The class is entirely marine (breathing by plume-like gills 

 on the sides of the body), and carnivorous. The naked 

 species are found in every sea. Those with chambered 

 shells (as Nautilus, Ammonites, and Orthoceras) were once 

 very abundant: more than two thousand fossil species are 

 known, but only one living representative the Pearly 

 Nautilus. 



1. Tetrahranchs. This order is characterized by the 

 possession of four gills, forty or more short tentacles, and 

 an external, chambered shell. The partitions, or septa, of 

 the shell are united by a tube called " siphuucle," and the 



Fi. 247. Pearly Nautilus, with shell bisected ; one half natural size. Indian Ocean. 



animal lives in the last and largest chamber. 14 * The liv- 

 ing Nautilus has a smooth, pearly shell, a head retractile 

 within the mantle or "hood," and calcareous mandibles, 

 well fitted for masticating Crabs, on which it feeds. This 



