8 GENERAL SURVEY OF THE ANIMAL KINGDOM. 



It does not matter much whether we retain the familiar title Verte 

 brata, or adopt that of Chordata, provided that we recognise ( I ) that 

 it is among Fishes first that separate vertebral bodies appear in the 

 supporting dorsal axis of the body ; (2) that, as a characteristic, the 

 backbone is less important than the notochord, which precedes it in 

 the history alike of the race and of the individual. Nor need we 

 object to the popular title backboned, if we recognise that the adjective 

 *' bony " is first applicable among Fishes, and not to all of these. 



The essential characters of Vertebrates may be summed up in the 

 following table, where they are contrasted, somewhat negatively, with 

 what is true of Invertebrates : 



INVERTEBRATES, OR BACKBONELESS ANIMALS 



Molluscs. If we take the concentration of the nervous 

 system as a useful criterion, the highest back boneless 

 animals are the Molluscs. This series of forms includes 

 Bivalves, such as cockle and mussel, oyster and clam; 

 Gasteropods, such as snail and slug, periwinkle and whelk ; 

 Cephalopods, such as octopus and pearly nautilus. 



Unlike Vertebrates, and such Invertebrates as Insects 

 and Crustaceans, Molluscs are without segments and 

 without appendages. A muscular protrusion of the ventral 

 surface, known as the " foot," serves in the majority as an 

 organ of locomotion. In most cases a single or double 

 fold of skin, called the " mantle," makes a protective shell. 

 The nervous system has three chief pairs of nerve centres 

 or ganglia. In many cases there are very characteristic 

 free-swimming larval stages. 



