io Vertebrata. 



papillae, which become converted into a very hard 

 kind of bones for the purpose of seizing and dividing 

 their food ; these are known as teeth. In higher 

 forms these become rooted in the subjacent bones, 

 but in all cases they arise as papillae of the mucous 

 membrane. 



The products of waste (which is constantly taking 

 place) are got rid of by means of certain purifying 

 organs. The skin, by means of its glands, removes 

 some of these effete matters ; so do certain areas of 

 the pharynx, richly supplied with blood-vessels from 

 the aortic arches, and which are called the respiratory 

 organs. There are also developed certain glandular 

 tubes in the hinder portion of the visceral cavity, of 

 the same nature, and built on the same plan, as the 

 segmental tubes of worms, which eliminate from the 

 blood the nitrogenised waste products ; these organs 

 are called kidneys. 



7. Primary and secondary segments. In the 

 body of a vertebrate animal there is to be seen the 

 remains of a primary segmentation into a chain of 

 successional divisions ; thus many organs or parts 

 are repeated in a series, such as the vertebrae, the 

 nervous system, the muscle masses (as can be seen 

 in fishes), and the tubes which constitute the kidney. 

 At the same time there is such a tendency to con- 

 centration noticeable that this segmental symmetry is 

 only to be seen in the lower forms, or in the embryonic 

 stages of the higher, secondary modes of aggregation 

 of parts masking completely the original systems of 

 segments. For example, while in the embryo the 

 primitive vertebrae can be distinguished clearly from 



