112 GENERAL BIOLOGY 



and rigidity. In development, this " backbone " is 

 preceded by an unjointed rod of cartilage called the 

 notochord, which is ultimately replaced by the verte- 

 brae. The very lowest types, however, do not 

 develop any other axial skeleton than the notochord. 

 The great group of animals possessing this structure, 

 either in the embryo or in the adult, is called the 

 Chordata. Thus, all vertebrates are chordates, but 

 some chordates are not vertebrates (i.e. have no true 

 backbone) . 



Exoskeleton. In many of the simpler forms of 

 animal life, instead of an internal skeletal frame- 

 work upon which the softer parts are suspended, there 

 is developed a thick and hard external skeleton, which 

 not only supports the internal organs, but protects 

 them as well. Particularly in the insects and the 

 Crustacea (crabs and their relatives) the outer skin 

 secretes a very tough, hard substance called chitin 

 which forms a sort of shell, jointed at appropriate 

 places to permit of free movement. The animal is 

 thus provided with a semipermanent suit of armor 

 which, being non-elastic, must be shed periodically 

 to permit of body growth. Many of the vertebrates 

 that do not require an exoskeleton for the purpose of 

 body support are provided with such skeletal struc- 

 tures for protection. Thus the scales of fishes arid 

 snakes are a sort of very flexible coat of mail, and 

 the feathers of birds and thick hair of wild animals 

 belong to the same category. In the turtles the 

 development of protective armor has gone on to such 



