394 THE WORLD OF ANIMAL LIFE 



This great sub-kingdom of animal forms is divided into five 

 classes as follows: — 



I. Cephalopoda — Cuttle-Fishes, Squids, Nautilus, &c. 



II. Gasteropoda (the Univalves) — Snails, Whelks, Periwinkles, &c. 



III. Amphineura — Chitons, and certain of their relatives. 



IV. Scaphopoda — The " tooth-shell " animals and their allies. 

 V. Pelecypoda (the Bivalves) — Mussels, Cockles, Oysters, &c. 



From our own knowledge we can see, by glancing at this 

 classification, that the three important classes among the five are 

 the Cephalopoda, the Gasteropoda, and the Pelecypoda. In 

 comparison with these, the third and fourth classes are small in 

 numbers, and unimportant. 



Probably four-fifths of all the Mollusca are inhabitants of water. 

 Many of them live in fresh water, but the greater number are found 

 in the sea. They are also, however, widely distributed over the 

 land surfaces of the world. 



As may be supposed from the enormous number of species, the 

 Mollusca are divided into a great number of orders, sub-orders, 

 families, and genera, but we need not trouble ourselves about these, 

 except to classify the few illustrative specimens which we shall 

 consider. 



CEPHALOPODA 



The Cephalopoda are the most highly developed of the Mol- 

 lusca. Except in very few instances they do not live in shells, 

 as the larger number of the Mollusca do; they may therefore be 

 styled a naked group. As a rule, however, they secrete a " shell " 

 inside their bodies, which in many cases forms a support to them 

 like the backbone among the Vertebrates. But it is a shell in all 

 cases, and not a bone, although it is at times horny in its nature. 

 It is generally of the material of which true shells are made, 

 namely, carbonate of lime and animal matter, and in some of the 



