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 Orthoeeras) were once 
very abundant: more than two thousand fossil species are 
known, but only one living representative — the Pearly 
Nautilus. 
1. Tetrdbranchs. — 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 “ siphuncle,” and the 
Fio. 247—Pearly Nautilus, with shell bisected ; one half natural size. Indian Ocean. 
animal lives in the last and largest chamber. 143 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 
