THE METAZOA 



7' 



walls of the blood-vessels themselves, or by the agency of a 

 special organ, the heart. The heart is essentially a sac with 

 muscular walls. Its cavity is in communication with the 

 main blood-vessels, and its walls contract regularly and 

 drive the blood through the system of vessels, the direction 

 of flow being regulated by a system of valves. 



The nitrogenous waste-matters which are produced as a 

 result of the chemical changes that accompany vital action 

 in the various organs, are separated out and got rid of by a 

 system of organs known as the organs of excretion, or renal 

 organs — this process of elimination being known as the 

 process of renal excretion. 



It is by means of the nervous system that the animal 

 receives impressions from the exterior and from the internal 

 organs, and that the various internal parts are brought into 

 vital communication with one another. The nervous system 

 extends as a complicated system of nerves or bundles of 

 nerve-fibres throughout all parts of the body. Large 

 aggregations of nerve-cells and nerve-fibres forming the 

 centres of the system are known as nerve-ganglia. When 

 one of these, or a group of them, situated towards the 

 anterior end, preponderates in size over the others, it 

 is termed the brain. Forming an important part of the 

 nervous system are the organs of the special senses, — 

 sight, hearing, smell, and taste, each of which is an organ 

 adapted for the reception of impressions of a special kind 

 from the exterior, — the impressions of light, of sound waves, 

 of the particles and substances that produce the sensations 

 of smell and taste. The less specialised sense of touch and of 

 heat and cold is diffused generally over the integument, in 

 which there are frequently special cells, or groups of cells, 

 with nerve-fibres terminating in them, that are concerned 

 with such sensations. 



